Display medium, display device and display method

ABSTRACT

A display medium includes at least: a pair of substrates, at least one of the substrates having optical transparency; a dispersion medium positioned in a gap between the pair of substrates; one or more kinds of electrophoretic particles or two or more kinds of electrophoretic particles different in color from each other, included in the dispersion medium; and a holder disposed between the pair of substrates, the holder having a function of holding the electrophoretic particles and a function of controlling, by an external voltage, a movement amount of the electrophoretic particles on the holder.

CROSS-REFERENCE TO RELATED APPLICATIONS

This application is based on and claims priority under 35 USC 119 fromJapanese Patent Applications No. 2006-305162 filed Nov. 10, 2006 and No.2006-339017 filed Dec. 15, 2006.

BACKGROUND

1. Technical Field

The invention relates to a display medium, a display device and adisplay method.

2. Related Art

As an environmentally conscious display medium, a display medium hasbeen known which displays by using an electrophoresis phenomenon causedwhen charged particles are placed in an electric field. The displaymedium has a memory property by which even in a state where an electricfield is not applied a displayed image can be maintained withoutdisappearance; accordingly, there is an advantage in that the powerconsumption is small. Furthermore, in order to make use of thisadvantage, it is important that a display medium is formed into areflective type to enable a particularly clear display.

As a display medium that enables a clear display, for example, a displaymedium has been known which has a configuration where between a pair ofsubstrates shielding spherical bodies that can shield light travelingfrom one substrate side toward the other substrate side or traveling inthe inverse direction are disposed and display particles can passthrough gaps of the spherical bodies. In the display medium, bydifferentiating optical properties between the spherical bodies and thedisplay particles, a clear display is obtained.

SUMMARY

According to an aspect of the invention, there is provided a displaymedium, including at least: a pair of substrates, at least one of thesubstrates having optical transparency; a dispersion medium positionedin a gap between the pair of substrates; one or more kinds ofelectrophoretic particles or two or more kinds of electrophoreticparticles different in color from each other, included in the dispersionmedium; and a holder disposed between the pair of substrates, the holderhaving a function of holding the electrophoretic particles and afunction of controlling, by an external voltage, a movement amount ofthe electrophoretic particles on the holder.

BRIEF DESCRIPTION OF THE DRAWINGS

Exemplary embodiments of the present invention will be described indetail based on the following figures, wherein:

FIGS. 1A and 1B are schematic diagrams for explaining a displayprinciple of a display method of an exemplary embodiment;

FIG. 2 is a schematic diagram for explaining a display principle of adisplay method of an exemplary embodiment;

FIGS. 3A and 3B are schematic diagrams for explaining a displayprinciple of a display method of an exemplary embodiment;

FIG. 4 is a schematic diagram for explaining a display principle of adisplay method of an exemplary embodiment;

FIGS. 5A and 5B are schematic diagrams for explaining a displayprinciple of a display method of an exemplary embodiment;

FIG. 6 is a schematic diagram for explaining a display principle of adisplay method of an exemplary embodiment;

FIGS. 7A and 7B are schematic diagrams for explaining a displayprinciple of a display method of an exemplary embodiment;

FIG. 8 is a schematic diagram showing an example of a display medium ofan exemplary embodiment;

FIG. 9 is a schematic diagram showing another example of a displaymedium of an exemplary embodiment;

FIG. 10 is a schematic diagram showing another example of a displaymedium of an exemplary embodiment;

FIG. 11 is a graph explaining relationship between threshold voltages ofthree kinds of electrophoretic particles used in a display medium shownin FIG. 10 and display densities thereof;

FIG. 12 is a schematic diagram showing an example of a display state inthe display medium shown in FIG. 10;

FIG. 13 is a schematic diagram showing another example of a displaystate in the display medium shown in FIG. 10;

FIG. 14 is a schematic diagram showing another example of a displaystate in the display medium shown in FIG. 10;

FIG. 15 is a schematic diagram showing another example of a displaystate in the display medium shown in FIG. 10;

FIG. 16 is a schematic diagram showing another example of a displaystate in the display medium shown in FIG. 10;

FIG. 17 is a schematic diagram showing another example of a displaystate in the display medium shown in FIG. 10;

FIG. 18 is a schematic diagram showing another example of a displaystate in the display medium shown in FIG. 10;

FIG. 19 is a schematic diagram showing another example of a displaystate in the display medium shown in FIG. 10;

FIG. 20 is a schematic diagram showing another example of a displaystate in the display medium shown in FIG. 10;

FIG. 21 is a schematic diagram showing another example of the displaymedium shown in FIG. 9; and

FIG. 22 is a schematic diagram showing another example of the displaymedium shown in FIG. 9.

DETAILED DESCRIPTION —Display Method—

The display method in the exemplary embodiment is a display method ofswitching a display by carrying out the following processes in anyorder, the method including:

applying an electric field to a light-modulating layer that includes adispersion medium, one or more kinds of electrophoretic particles or twoor more kinds of electrophoretic particles different in color from eachother, included in the dispersion medium, and a holder having a functionof holding the electrophoretic particles (hereinafter referred to as an“electrophoretic particle holder”), the electric field forming apotential gradient and moving the electrophoretic particles via theelectrophoretic particle holder, to localize the electrophoreticparticles at one side of the light-modulating layer, thereby displayinga color due to the electrophoretic particles at the one side of thelight-modulating layer at a maximum density;

applying an electric field to the light-modulating layer, the electricfield forming a potential gradient and moving the electrophoreticparticles via the electrophoretic particle holder, to localize theelectrophoretic particles at the other side of the light-modulatinglayer, thereby displaying a color due to the electrophoretic particlesat the one side of the light-modulating layer at a minimum density; and

applying an electric field to the light-modulating layer, the electricfield forming a potential gradient and moving the electrophoreticparticles via the electrophoretic particle holder, to localize theelectrophoretic particles between the one side and the other side of thelight-modulating layer, thereby displaying a color due to theelectrophoretic particles at the one side of the light-modulating layerat a density smaller than the maximum density but larger than theminimum density.

Now, the electrophoretic particles and electrophoretic particle holder,which are used in the exemplary embodiment, form an electric doublelayer in a dispersion medium, and there is a case where one of these ispositively charged and the other is negatively charged or a case whereonly the electrophoretic particles are charged. Furthermore, as theelectrophoretic particles and electrophoretic particle holder used inthe invention, those of which charging voltages are designed so as tosatisfy the following characteristics may be used.

That is, when the electrophoretic particles are located on a surface ofan electrophoretic particle holder or in the neighborhood thereof, byusing a force working between bodies (mainly an electrostatic force) ora steric hindrance due to a structure of the electrophoretic particleholder, the electrophoretic particles are held on a surface of theelectrophoretic particle holder. However, when the electrophoreticparticle holder on a surface of which the electrophoretic particles areheld is placed in an electric field equal to or higher than apredetermined electric field strength, the electrophoretic particlesovercome an electrostatic adsorption force or the steric hindrance andthereby can escape from a state of being held on a surface of theelectrophoretic particle holder and move.

In order to control the adherence between the electrophoretic particlesand the electrophoretic particle holder (that is, a holding function),as a force that works between bodies, other than the electrostaticforce, a magnetic force or an intermolecular force may be used.

The holding of the electrophoretic particles by using the sterichindrance means a state where, though depending on a configuration ofthe electrophoretic particle holder (which will be detailed later),owing to members such as fibers or particles that constitute theelectrophoretic particle holder, the electrophoretic particles areinhibited from moving on a surface of the electrophoretic particleholder.

Here, “a surface of an electrophoretic particle holder” means a regionof an electrophoretic particle holder abutting on a boundary between theelectrophoretic particle holder and the outside of the electrophoreticparticle holder, and means an outer circumferential surface of theelectrophoretic particle holder and, when the electrophoretic particleholder has pores, inner circumferential surfaces of the pores.

Furthermore, the “maximum density” means a state where, owing to anelectric field, 95% or more, specifically 98% or more of one kind of theparticles are localized at a display surface side and thereby a color ofthe particles can be seen from a display surface.

Specifically, the maximum density means a density when, while the colordensity at one side of a light-modulating layer is measured with areflection densitometer (manufactured by X-rite Corp.,) as an opticaldensity (OD), a voltage is applied between the one side and the otherside of the light-modulating layer and gradually varied so as toincrease the measurement density (by increasing or decreasing an appliedvoltage value), so that a density variation per unit voltage issaturated and, even when in this state a voltage and a voltageapplication time are further increased, the density is not varied andthus saturated.

Furthermore, the “minimum density” means a state where, owing to anelectric field, 95% or more, specifically 98% or more of one kind ofparticles are localized at a side opposite to the display surface andthereby a color of the particles cannot be seen from a display surface.

Specifically, the minimum density means a density when, while the colordensity at one side of a light-modulating layer is measured with areflection densitometer (manufactured by X-rite Corp.,) as an opticaldensity (OD), a voltage is applied between the one side and the otherside of the light-modulating layer and gradually varied so as todecrease the measurement density (by decreasing or increasing an appliedvoltage value), so that a density variation per unit voltage issaturated and, even when a voltage and a voltage application time arefurther increased at this state, the density is not varied and thussaturated.

The electrophoretic particle holder has a function of sticking andholding electrophoretic particles on a surface (at least on an outercircumferential surface or inner circumferential surface) thereof. The“holding” means that an sticking force works between the surface of theelectrophoretic particle holder and the electrophoretic particles to anextent wherein the electrophoretic particles are not detached from thesurface of the electrophoretic particle holder due to gravity. Morespecifically, the “holding” means that, in a display device using adisplay method of the exemplary embodiment, when, after an electricfield is applied to a light-modulating layer to form an image, a stateof stopping an electric field application to the light-modulating layeris continued, a variation in the reflectance 10 days after the imageformation is within±5% based on the reflectance at the formation of theimage as a reference (100%).

Furthermore, the polarity and charging potential of the electrophoreticparticles and the electrophoretic particle holder may be readilycontrolled by selecting main components included in these members andoptional additives such as a charge control agent and a dispersant, sothat the above-described characteristics may be obtained. Still further,the electrophoretic particle holder should not be electrophoresed whenan electric field is applied to a light-modulating layer. For example,an electrophoretic particle holder can be fixed at a predeterminedposition in a light-modulating layer so as not to be moved, or can beconstituted with a member having a weight so as not to be moved evenwhen an electric field is applied. The electrophoretic particles and theelectrophoretic particle holder will be detailed later.

Accordingly, in the case where an electric field is applied to thelight-modulating layer, when the electric field has an electric fieldstrength that is capable of detaching the electrophoretic particles heldon a surface of the electrophoretic particle holder from the surface ofthe electrophoretic particle holder to move in a dispersion medium, theelectrophoretic particles can move along a direction of a potentialgradient in the light-modulating layer.

On the other hand, when an electric field applied to thelight-modulating layer has an electric field strength that can notdetach the electrophoretic particles held on a surface of theelectrophoretic particle holder from the surface of the electrophoreticparticle holder or when an electric field is not applied to thelight-modulating layer, the electrophoretic particles continue to be ina state of being held on a surface of the electrophoretic particleholder. Accordingly, when a display method of the exemplary embodimentis utilized, an excellent sustainability of a display state can beobtained.

Furthermore, by selecting a strength of an applied electric field or atime period during which an electric field is applied, not only themaximum density and the minimum density can be displayed (first andsecond display processes) similarly to an conventional display medium,but also an arbitrary density between the maximum density and theminimum density, that is, a halftone can be displayed (a third displayprocess) in each of the pixels.

Explanation of the “maximum density” and “minimum density”, which is thesame as the definitions described above, are omitted here.

In the next place, a display principle of a display method of theexemplary embodiment will be detailed with reference to the drawings.FIGS. 1A through 7B are schematic diagrams for describing a displayprinciple of a display method of the exemplary embodiment and show amovement of electrophoretic particles present on a surface of oneparticulate electrophoretic particle holder and in the neighborhoodthereof or sticking positions on a surface of the electrophoreticparticle holder.

In the drawings, reference numerals 10, 20, 30, 32, 34, 40, 42, 50, 52and 60, respectively, represent an electrophoretic particle, anelectrophoretic particle holder, a light-modulating layer, one side ofthe light-modulating layer 30, the other side of the light-modulatinglayer 30, a transparent substrate (a substrate having opticaltransparency), a substrate, a transparent electrode, an electrode and anelectric field applicator.

In embodiments shown in the drawings, it is assumed that theelectrophoretic particles 10 are positively charged, the electrophoreticparticle holder 20 is negatively charged, and an electric field appliedon the light-modulating layer 30 by use of the electric field applicator60 has an electric field strength capable of detaching theelectrophoretic particles 10 electrostatically stuck to and held on asurface of the electrophoretic particle holder 20 from the surface ofthe electrophoretic particle holder 20 to move in a dispersion medium.Furthermore, the electrophoretic particle holder 20 is fixed in thelight-modulating layer 30 so as not to move even when an electric fieldis applied, and colored in a color different from that of theelectrophoretic particles 10.

Here, FIG. 1A shows a state where electrophoretic particles 10 arelocalized at the other side 34 of the light-modulating layer 30; FIG. 1Bshows a state of the electrophoretic particle holder 20 shown in FIG. 1Aobserved from one side 32 of the light-modulating layer 30; FIG. 2 showsa state where, in a state shown in FIG. 1A, an electric field is appliedto the light-modulating layer 30 with the transparent electrode 50 setminus and the electrode 52 set plus, and the electrophoretic particles10 move from the other side 34 of the light-modulating layer 30 to oneside 32 of the light-modulating layer 30; FIG. 3A shows a state where anelectric field application shown in FIG. 2 is finished, and theelectrophoretic particles 10 are localized between one side 32 of thelight-modulating layer 30 and the other side 34 of the light-modulatinglayer 30; FIG. 3B shows a state where the electrophoretic particleholder 20 shown in FIG. 3A is observed from one side 32 of thelight-modulating layer 30; and FIG. 4 shows a state where, in a stateshown in FIG. 3A, an electric field is applied to the light-modulatinglayer 30 with the transparent electrode 50 set minus and the electrode52 set plus, and the electrophoretic particles 10 move from the otherside 34 of the light-modulating layer 30 to one side 32 of thelight-modulating layer 30.

Furthermore, FIG. 5A shows a state where an electric field applicationshown in FIG. 4 is finished, and the electrophoretic particles 10 arelocalized between one side 32 of the light-modulating layer 30 and theother side 34 of the light-modulating layer 30; FIG. 5B shows a statewhere the electrophoretic particle holder 20 shown in FIG. 5A isobserved from one side 32 of the light-modulating layer 30; FIG. 6 showsa state where, in a state shown in FIG. 5A, an electric field is appliedto the light-modulating layer 30 with the transparent electrode 50 setminus and the electrode 52 set plus and thereby the electrophoreticparticles 10 move from the other side 34 of the light-modulating layer30 to one side 32 of the light-modulating layer 30; FIG. 7A shows astate where an electric field application shown in FIG. 6 is finished,and the electrophoretic particles 10 are localized at one side 32 of thelight-modulating layer 30; and FIG. 7B shows a state where theelectrophoretic particle holder 20 shown in FIG. 7A is observed from oneside 32 of the light-modulating layer 30.

As shown in FIGS. 1A through 7B, the light-modulating layer 30 isdisposed between a pair of transparent substrates 40 and 42 disposedfacing to each other, a side of the light-modulating layer 30 at whichthe transparent substrate 40 is disposed represents one side 32 of thelight-modulating layer 30, and a side of the light-modulating layer 30at which the substrate 42 is disposed represents the other side 34 ofthe light-modulating layer 30. Furthermore, the light-modulating layer30 contains electrophoretic particle holder 20 having a diameter equalto a gap between the transparent substrate 40 and substrate 42,electrophoretic particles 10 having a diameter smaller than that of theelectrophoretic particle holder 20, and a dispersion medium (not shownin the drawings).

Furthermore, on a surface of the transparent substrate 40 where thesubstrate 42 is disposed, a transparent electrode 50 is disposed, on asurface of the substrate 42 where the transparent substrate 40 isdisposed, an electrode 52 is disposed, and the pair of transparentelectrodes 50 and 52 are connected to an electric field applicator 60.Accordingly, when an electric field is applied to the light-modulatinglayer 30, a potential gradient is formed in a direction from one side 32of the light-modulating layer 30 to the other side 34 of thelight-modulating layer 30 (or in an opposite direction thereof).

In the next place, a display operation will be described. In thebeginning, in a state shown in FIGS. 1A and 1B, the electrophoreticparticles 10 are localized at the other side 34 of the light-modulatinglayer 30; accordingly, when the electrophoretic particle holder 20 isobserved from one side 32 of the light-modulating layer 30, theelectrophoretic particles 10 are hidden by the electrophoretic particleholder 20. As a result, when the light-modulating layer 30 is observedfrom the transparent substrate 40 side, the color density due to theelectrophoretic particles 10 becomes the minimum density.

Subsequently, when an electric field is applied as shown in FIG. 2, theelectrophoretic particles 10 move from the other side 34 of thelight-modulating layer 30 to one side 32 of the light-modulating layer30 along a surface of the electrophoretic particle holder 20 or theneighborhood thereof and, when the electric field application isstopped, a state shown in FIGS. 3A and 3B is obtained.

In a state shown in FIGS. 3A and 3B, the electrophoretic particles 10are localized in a position between one side 32 of the light-modulatinglayer 30 and the other side 34 of the light-modulating layer 30 andcloser to the other side 34 of the light-modulating layer 30.Accordingly, when the electrophoretic particle holder 20 is observedfrom one side 32 of the light-modulating layer 30, although a large partof the electrophoretic particles 10, being hidden by the electrophoreticparticle holder 20, cannot be observed, some of the electrophoreticparticles 10 held on a surface of the electrophoretic particle holder 20are observed. Accordingly, when the light-modulating layer 30 isobserved from the transparent substrate 40 side, the color density dueto the electrophoretic particles 10 becomes a density slightly higherthan the minimum density (display of halftone).

Subsequently, an electric field is applied as shown in FIG. 4, theelectrophoretic particles 10 move from the other side 34 of thelight-modulating layer 30 to one side 32 of the light-modulating layer30 along a surface of the electrophoretic particle holder 20 or theneighborhood thereof and, when the electric field application isstopped, a state shown in FIGS. 5A and 5B is obtained.

In a state shown in FIGS. 5A and 5B, the electrophoretic particles 10are localized in a region between one side 32 of the light-modulatinglayer 30 and the other side 34 of the light-modulating layer 30 andcloser to one side 32 of the light-modulating layer 30. Accordingly,when the electrophoretic particle holder 20 is observed from one side 32of the light-modulating layer 30, although some of the electrophoreticparticles 10 are hidden by the electrophoretic particle holder 20 andcannot be observed, a large part of the electrophoretic particles 10held on a surface of the electrophoretic particle holder 20 areobserved. Accordingly, when the light-modulating layer 30 is observedfrom the transparent substrate 40 side, the color density due to theelectrophoretic particles 10 becomes a density higher than the colordensity in a state shown in FIGS. 3A and 3B (display of halftone).

Subsequently, an electric field is applied as shown in FIG. 6, theelectrophoretic particles 10 move from the other side 34 of thelight-modulating layer 30 to one side 32 of the light-modulating layer30 along a surface of the electrophoretic particle holder 20 or theneighborhood thereof and, when the application of electric field isstopped, a state shown in FIGS. 7A and 7B is obtained.

In a state shown in FIGS. 7A and 7B, the electrophoretic particles 10are localized at one side 32 of the light-modulating layer 30;accordingly, when the electrophoretic particle holder 20 is observedfrom one side 32 of the light-modulating layer 30, almost all of theelectrophoretic particles 10 held on a surface of the electrophoreticparticle holder 20 are confirmed. Accordingly, when the light-modulatinglayer 30 is observed from the transparent substrate 40 side, the colordensity due to the electrophoretic particles 10 becomes the maximumdensity.

On the other hand, as described above, by applying, to alight-modulating layer, an electric field stronger than the electricfield capable of detaching the electrophoretic particles in a state ofbeing held on a surface of the electrophoretic particle holder from asurface of the electrophoretic particle holder, one display state can beswitched to the other display state. Here, a range of the electric fieldstrength necessary for switching one display state to the other displaystate, though not particularly restricted, may be set to a value in adefinite range, considering a power source that can be practically usedand a thickness of the light-modulating layer suitable for applicationsof a display medium using a display method of the exemplary embodiment.

From the above viewpoints, an absolute value of a threshold value of anelectric field by which the electrophoretic particles held on a surfaceof the electrophoretic particle holder are detached from a surface ofthe electrophoretic particle holder and go through a dispersion mediummay be in the range of 100 V/cm to 30 kV/cm, and particularly in therange of 300 V/cm to 10 kV/cm.

When the absolute value of the threshold value of the electric field isless than 100 V/cm, in some cases, the memory property may bedeteriorated, and when the absolute value of the threshold value of theelectric field exceeds 30 kV/cm, in some cases, the energy consumptionnecessary for switching the display may be increased excessively.

The “threshold value of an electric field” is as follows. That is, whenthe absolute value of the electric field applied to a light-modulatinglayer is equal to or higher than the absolute value of the thresholdvalue of the electric field (V/m), the electrophoretic particles can bedetached from a surface of the electrophoretic particle holder to movein a dispersion medium and, in the case contrary to the above, theelectrophoretic particles maintain a state of being held on a surface ofthe electrophoretic particle holder.

Furthermore, in a display method of the exemplary embodiment, as to anelectric field applied to the light-modulating layer, a voltage value, avoltage waveform and an application time of the electric field may beset so as to enable to display a halftone. Here, as the voltagewaveform, practically, one having a rectangular waveform may be used.However, without restricting the voltage waveform to the rectangularwaveform, for example, one having a sine waveform or an irregularwaveform may be used.

When in the invention an electric field is applied to a light-modulatinglayer, a potential gradient direction to the light-modulating layer maybe always constant and electrodes used to apply the electric field tothe light-modulating layer may be disposed so as to always maintain aconstant distance in the potential gradient direction. In this case,since a voltage (V) of an electric field applied to the light-modulatinglayer and the electric field strength (V/cm) are proportional, there isa threshold voltage (V) corresponding to a threshold value of anelectric field (V/cm).

On the other hand, when the display is carried out while switching adisplay state, a voltage waveform of an electric field applied to thelight-modulating layer has cycles each including a first interval wherean electric field is continuously applied at a voltage where an absolutevalue of the voltage is equal to or higher than an absolute value of athreshold voltage corresponding to a threshold value of an electricfield and a second interval where an electric field is continuouslyapplied at a voltage where an absolute value of the voltage is less thanan absolute value of a threshold voltage corresponding to a thresholdvalue of an electric field (wherein, the second interval may include astate where a voltage is 0 V, that is, an electric field is notapplied).

When a voltage in the first interval in an a-th cycle (“a” means aninteger of 1 or larger) is positive and a voltage in the first intervalin an (a+1)-th cycle is negative, a continuation time of the secondinterval between the first interval in the a-th cycle and the firstinterval in the (a+1)-th cycle may be set at 0.

Here, when a halftone is displayed, a waveform in the first interval inat least any one of the cycles may be set so as to satisfy the followingformula (5):

Ep<Emax   Formula (5)

wherein Ep represents a value represented by the following formula (6),and Emax means a product of voltage·time (V·s) necessary for varying thedisplay density from the maximum density to the minimum density or fromthe minimum density to the maximum density when an electric field iscontinuously applied to the light-modulating layer at a voltage where anabsolute value of the voltage is equal to or higher than an absolutevalue of the threshold voltage

E _(p)=∫₀ ^(t) ^(p) V(t)dt   Formula (6)

wherein t represents an arbitrary time (s) in the first interval in theat least any one of the cycles, tp represents a time (s) from a start toan end of the first interval in the at least any one of the cycles, andV (t) represents a voltage (V) at the time t.

When, as the electrophoretic particles, two or more kinds ofelectrophoretic particles different from each other in the thresholdvalue of the electric field are used, the formula (5) can be appliedbased on the threshold voltage corresponding to a threshold value of anelectric field of at least any one kind of the electrophoreticparticles, the minimum density, and the maximum density.

Furthermore, a calculation method of a product (Emax) of voltage andtime shown in the formula (5) is not particularly restricted. However,for example, when an electric field is continuously applied at anaverage voltage Vave (from t=0 to tp) of a voltage value V(t) in thefirst interval in the at least any one of the cycles, a time tcnecessary for varying a display state from the maximum density to theminimum density or vice versa is determined and thereby a product of theVave and the tc is obtained.

However, practically, a voltage waveform of an electric field applied tothe light-modulating layer may be one that maintains a constant voltage(first voltage) at least in the first interval. Furthermore, it may beone that maintains a constant voltage (second voltage) in the secondinterval (that is, rectangular voltage waveform).

In the case where a voltage waveform is one that maintains a constantvoltage (first voltage) at least in the first interval, when a time Tpduring which an electric field is applied to the light-modulating layerat the first voltage satisfies the following formula (7) in a firstinterval in at least any one of the cycles, a halftone can be displayed:

Tp<Tmax   Formula (7)

wherein Tp represents a time (s) during which an electric field isapplied to the light-modulating layer at a first voltage (V) in a firstinterval in the at least any one of the cycles, and Tmax represents atime (s) necessary for varying the display density from the maximumdensity to the minimum density or from the minimum density to themaximum density when an electric field is continuously applied at thefirst voltage (V) to the light-modulating layer.

A value of the Tp may be set so as to satisfy the following formula (8):

$\begin{matrix}{T_{\max} = {\sum\limits_{a = 1}^{n}{T_{p}(a)}}} & {{Formula}\mspace{14mu} (8)}\end{matrix}$

wherein n denotes an integer of 2 or larger, a denotes an integer of 1to n, and as the value increases, the display density (due to any onekind of the electrophoretic particles when two or more kinds ofelectrophoretic particles are used) varies in a direction from theminimum density side to the maximum density side (or vice versa), and Tp(a) represents a time (s) during which an electric field is applied tothe light-modulating layer at a first voltage (V) and a value that cantakes a value exceeding zero but less than the Tmax.

When the formula (8) is satisfied, even in one pixel unit, gradationdisplay of (n+1) levels can be realized. Here, two levels mean thatthere are two display densities (for example, white state and blackstate) of the minimum density and the maximum density, and four levelsmean that there are two different intermediate density states betweenthe maximum density and the minimum density.

Here, the “intermediate density” fundamentally indicates the reflection(optical) density (measurement is carried out with X-rite 404(manufactured by X-rite Corp.,) similarly to the above) and indicates astate where a plurality of density states separated with substantiallyequal intervals is present between the maximum density and the minimumdensity.

Here, the respective values of Tp(1) . . . Tp(a) . . . , Tp(n) are notnecessarily the same. A variation in the display density when a displaystate is transferred from an a-th display state to an (a+1)-th displaystate and a variation in the display density when a display state istransferred from the (a+1)-th display state to an (a+2)-th display statemay be set visually equal.

Furthermore, when, in the case where a display state other than themaximum density and the minimum density (a display state other than the1st and the (n+1)-th display state) is displayed, a display state isswitched to an initial state (the maximum density or minimum density), atime during which an electric field is applied at the first voltage isnot particularly restricted as far as it is equal to or longer than anecessary minimum time for switching a display state to the initialstate.

When the formula (8) is satisfied, n, though being necessarily 2 ormore, may be 4 or more. The upper limit thereof, without particularlyrestricted, may be 256 or less, particularly 64 or less. When the numberof levels is increased excessively, a large memory amount for displaydrive becomes necessary, thus raising part cost and making a circuitboard larger. Furthermore, it takes a long time for processing imagedata for representing gradations, so that a waiting time up to displaybecomes longer.

—Display Medium—

In the next place, a display medium that makes use of a display methodof the exemplary embodiment will be described. The display medium of theexemplary embodiment, without particularly restricted as far as it has aconfiguration where the display method of the invention can beimplemented, may have a configuration below.

That is, the display medium of the exemplary embodiment includes atleast a pair of substrates at least one of which has opticaltransparency; a dispersion medium positioned in a gap between the pairof substrates; one or more kinds of electrophoretic particles or two ormore kinds of electrophoretic particles different in color from eachother, included in the dispersion medium; and an electrophoreticparticle holder that is disposed between the pair of substrates and havea function of sticking and holding the electrophoretic particles.

The electrophoretic particle holder has a function of controlling, byuse of an external voltage, a movement amount of electrophoreticparticles on the electrophoretic particle holder.

The “movement amount” indicates, in the exemplary embodiment, a movementamount when the electrophoretic particles are detached from a surface ofan electrophoretic particle holder, that is, an outer circumferentialsurface of the electrophoretic particle holder, or, when theelectrophoretic particle holder has pores, a state of being held in anarbitrary region such as an inner circumferential surface of the pore,and moved in a dispersion medium to a state of being held in otherregion on a surface of the electrophoretic particle holder or a regiondistanced from the electrophoretic particle holder. Specifically, the“movement amount” is represented by an absolute value of a thresholdvalue of an electric field at which the electrophoretic particles heldon a surface of the electrophoretic particle holder are detached from asurface of the electrophoretic particle holder and moved in thedispersion medium. The “absolute value of a threshold value of anelectric field at which the electrophoretic particles held on a surfaceof the electrophoretic particle holder are detached from the surface ofthe electrophoretic particle holder and moved in the dispersion medium”may satisfy the above-mentioned range.

The display medium of the exemplary embodiment, having a configurationcapable of carrying out the display method of the exemplary embodiment,is excellent in the memory property and can display a halftone as well.However, when the halftone is displayed, an electric field of whichvoltage waveform is controlled so as to be able to display the halftone(for example, an electric field having a waveform satisfyingabove-mentioned formula (5), (7) or (8)) is applied. In this case, anexternal electric field applicator capable of applying an electric fieldhaving the voltage waveform may well be used. However, when the displaymedium of the exemplary embodiment is used to display, an electric fieldsuitable for displaying a halftone is not necessarily applied, and anelectric field suitable for analog display may be applied.

When an electric field is applied to a member located between a pair ofsubstrates that is included in a display medium (dispersion medium,electrophoretic particles and electrophoretic particle holder), that is,when an electric field is applied to a light-modulating layer, in thecase where the display medium includes a pair of electrodes, the pair ofelectrodes can be used to apply the electric field, and, in the casewhere the display medium does not include a pair of electrodes, externalelectrodes can be used. However, since only connecting to an externalpower source enables to use the display medium, a pair of electrodes maybe included in a display medium. In this case, electrodes that are usedto apply an electric field to the light-modulating layer may be disposedon surface sides of the pair of substrates at which the substrates faceeach other, or, at a substrate (transparent substrate) side havingoptical transparency of the pair of substrates, an electrode havingoptical transparency (transparent electrode) may be disposed.

The display medium of the exemplary embodiment may include one or morekinds of electrophoretic particles in a dispersion medium. However,electrophoretic particles contained in the dispersion medium may beconstituted of two or more kinds of electrophoretic particles differentfrom each other in a color formed in a state of being dispersed in thedispersion medium and in an absolute value of a threshold value of anelectric field at which electrophoresis can be realized in a dispersionmedium. In this case, color display can be realized.

Furthermore, from the viewpoint of enabling a practical color display,the two or more kinds of electrophoretic particles may be constituted ofelectrophoretic particles that form a red color, electrophoreticparticles that form a green color and electrophoretic particles thatform a blue color, in a state of being dispersed in a dispersion medium.

Here, “one or more kinds of electrophoretic particles are contained in adispersion medium” means that, when a dispersion medium positionedbetween a pair of substrates is divided into a plurality of cells withpartition walls, the one or more kinds of electrophoretic particles arecontained in each dispersion medium of the cells.

On the other hand, in the case where a practical color display iscarried out when in a gap between a pair of substrates partition wallsare disposed to partition the gap to dispose cells in each of which adispersion medium containing one kind of electrophoretic particles ispositioned, the cells may be constituted of three kinds of cells, thatis, a cell (R cell) in which a dispersion medium containingelectrophoretic particles that form a red color in a dispersed state ina dispersion medium is positioned, a cell (G cell) in which a dispersionmedium containing electrophoretic particles that form a green color in adispersed state in a dispersion medium is positioned, and a cell (Bcell) in which a dispersion medium containing electrophoretic particlesthat form a blue color in a dispersed state in a dispersion medium ispositioned.

In this case, with three cells (3 pixels) constituted of an R cell, a Gcell and a B cell as one pixel, color display can be controlled. Thethree cells are disposed adjacent to each other or neighboring to eachother.

—Electrophoretic Particle Holder—

The electrophoretic particle holder, as far as it has a function ofsticking and holding electrophoretic particles on a surface thereof, isnot particularly restricted. However, inorganic particles constituted ofa material such as titanium oxide and zinc oxide, particulate members(electrophoretic particle holder particles) such as organic particlesconstituted of a material such as a methyl methacrylate resin, astyrene-acryl resin, a silicone resin and a polytetrafluoroethyleneresin, porous bodies such as gelatin and porous silica, networkstructures such as polymers having a network structure such aspolyacrylamide and aggregates of fibers (in a state where independentcord-shaped materials are regularly or irregularly entangled) may beused.

In order that the charging polarity of electrophoretic particles usedand the adherence thereof to the electrophoretic particles may realize agood balance in the sustainability of a display state and the display ofthe halftone, a material that constitutes the electrophoretic particleholder may be selected, or a surface treatment may optionally be appliedto the electrophoretic particle holder.

That is, a function of sticking and holding electrophoretic particlescan be realized as follows. That is, as mentioned above, a material thatconstitutes an electrophoretic particle holder is selected, a surfacetreatment is optionally applied to the electrophoretic particle holder,the charging polarity and charging amount are controlled so that theholder is at least partially charged with a polarity opposite to that ofall kinds of the electrophoretic particles, or the porosity, theaperture ratio of the pores and a diameter of the pore are controlled.

When an electrophoretic particle holding particle is used as anelectrophoretic particle holder, between a pair of substrates, at leasttwo or more particulate members (electrophoretic particle holdingparticles) may be disposed. However, practically, the electrophoreticparticle holding particles may be packed at a density to an extent wherethe electrophoretic particle holding particles cannot move from eachother. Furthermore, the electrophoretic particle holding particles maybe disposed in a state of being fixed on a surface of at least any oneof the substrates by laminating in one or more layers and using heatsealing. Optionally, a particulate member that does not have a functionas the electrophoretic particle holder may be mixed thereto. However,fundamentally, the electrophoretic particle holding particles alone maybe used.

An average particle diameter of the electrophoretic particle holdingparticles is not particularly restricted. However, the electrophoreticparticle holding particles may have an average particle diameter suchthat, when the particles are packed between a pair of substrates ordisposed by laminating on a surface of a substrate, through gaps of theelectrophoretic particle holding particles adjacent to each other, theelectrophoretic particles can pass.

Accordingly, an average particle diameter of the electrophoreticparticle holding particles may be ten times or more of an averageparticle diameter of all kinds of the electrophoretic particles or 25times or more. When the average particle diameter of the electrophoreticparticle holding particles is less than ten times of an average particlediameter of all kinds of the electrophoretic particles, since theelectrophoretic particles cannot go through the gaps between theelectrophoretic particle holding particles adjacent to each other, insome cases, a display state can be switched with difficulty. The upperlimit of the average particle diameter of the electrophoretic particleholding particles is not particularly restricted. However, it may beequal to or less than a distance between a pair of substrates (athickness of a light-modulating layer).

An average particle diameter of the electrophoretic particle holdingparticles is obtained in such a manner that electrophoretic particleholding particles used in a display medium are observed with a SEM or aTEM, and, based on the SEM image or TEM image, from areas of tenparticles, an average particle diameter is obtained. An average particlediameter of all kinds of the electrophoretic particles is obtained insuch a manner that an average particle diameter of each kind of theelectrophoretic particles is obtained from areas of ten particlessimilarly to the above, and then the sum of the each average particlediameter is divided by the number of kinds of the electrophoreticparticles to obtain the average particle diameter.

Here, when a dispersion medium positioned between a pair of substratesis partitioned into a plurality of cells with partition walls, parametervalues relating to particle shapes and sizes such as average particlediameters of the electrophoretic particle holder particles and all kindsof the electrophoretic particles mean values obtained in a cell unit.

On the other hand, when, as an electrophoretic particle holder, a porousbody, a network structure or an aggregate of fibers is used, thesemembers can be disposed in a state of being filled between a pair ofsubstrates or in a state of being fixed on a surface of at least any oneof substrates by heat sealing.

An average pore diameter of a porous body, a network structure or afiber aggregate, as far as it is a size in which all kinds of theelectrophoretic particles can move in the member when an electric fieldis applied, is not particularly restricted. However, an average porediameter of the porous body, network structure or fiber aggregate may be5 times or more or 10 times or more of an average particle diameter ofelectrophoretic particles of the kind of which average particle diameteris the largest. When the average pore diameter of the porous body,network structure or fiber aggregate is less than 10 times of an averageparticle diameter of all kinds of the electrophoretic particles, sincethe electrophoretic particles cannot pass through pores of the porousbody, network structure or fiber aggregate, in some cases, a displaystate can be switched with difficulty. The upper limit of the averagepore diameter of the porous body, network structure or fiber aggregateis not particularly restricted. However, when it is excessively large,since a surface area of the porous body, network structure or fiberaggregate in a unit volume becomes excessively small, a halftone may bedisplayed with difficulty or a holding function of electrophoreticparticles may be deteriorated; accordingly, an average pore diameter ofthe porous body, network structure or fiber aggregate may be 100 μm orless.

An average pore diameter of the porous body, network structure or fiberaggregate is obtained from SEM observation of a section of the members.An average pore diameter is obtained by measuring pore diameters ofarbitrary 100 points of pores observed in a section of the member,followed by averaging the pore diameters at the respective points.

Furthermore, the porosity of an electrophoretic particle holderconstituted of the porous body, network structure or fiber aggregate maybe in the range of 20 to 60%. In this case, the electrophoreticparticles of the number necessary for display may be contained, adesired holding force may be maintained, and the coloringcharacteristics of the holder may be sufficiently exerted. Moreparticularly, it may be in the range of 30 to 50%.

As the fiber aggregate, a block body in which fibers are simplygathered, one in which fibers are densely arranged, textile-like one inwhich threads obtained by twisting fibers are knit, net-shaped one orfabric-like one obtained by weaving, non-woven fabric one obtained bypartially melting or entangling fibers, web-shaped one and sheet-likeone may be exemplified.

Examples of the fiber aggregates include a non-woven fabric, a polymerfilm, cloth and paper. Among these, a non-woven fabric may be used. Inthe case of the non-woven fabric, since a fiber diameter and aninter-fiber distance may be independently designed, a function ofholding electrophoretic particles may be readily adjusted.

As fibers that constitute a fiber aggregate, for example, polyethylene,polystyrene, polyester, polyacryl, polypropylene and a fluororesin suchas polytetrafluoroethylene (PTFE) can be applied. Because of readinessof charging by a corona treatment, polypropylene and PTFE fibers may beused.

Furthermore, the density of the fiber aggregate, from reasons ofmaintaining a desired holding force to electrophoretic particles of thenumber necessary for display, and of maintaining the physical strengththereof, may be in the range of 10 to 70 g/m², and particularly in therange of 20 to 50 g/m².

Diameters of fibers that constitute the fiber aggregate may be in therange of 0.1 to 20 μm and particularly in the range of 0.1 to 3 μm. Inthis case, a sufficient surface area and physical strength may besecured.

When the porous body, network structure or fiber aggregate is used asthe electrophoretic particle holder, by controlling the density,porosity, average opening diameter and the electric characteristicsthereof, a holding force of the electrophoretic particles may becontrolled.

A color of the electrophoretic particle holder, as far as it is a colordifferent from that of the electrophoretic particles, is notparticularly restricted. However, usually, the electrophoretic particleholder may be colored.

The electrophoretic particle holder may be transparent. However, in thiscase, for example, it is necessary to use the electrophoretic particleholding particles mixed with colored particulate members that do nothave a function as the electrophoretic particle holder. This is becausewhen the electrophoretic particle holder is transparent, a function ofshielding light therewith to hide electrophoretic particles localized ata side of a substrate that is not transparent of a pair of substrateswhen seen from a transparent substrate side (hiding function) is lacked,and thereby, even when a third display process is carried out, ahalftone display cannot be achieved. Accordingly, when a transparentelectrophoretic particle holder is used, another member that has thehiding function is necessarily used.

The electrophoretic particle holder may have a white color. In thiscase, after a second display process has been carried out, a white colorcan be displayed.

The polarity and charging properties of the electrophoretic particleholder may be controlled with a primary material itself that constitutesthe electrophoretic particle holder. However, a charge control agent mayoptionally be added.

As the charge control agent, for example, known ones that are used for,for example, electrophotography toners can be used. Examples thereofinclude quaternary ammonium salts such as cetylpyridyl chloride, BONTRONP-51, BONTRON P-53, BONTRON E-84 and BONTRON E-81 (trade name, allmanufactured by Orient Chemical Industries, Ltd.), salicylic acid metalcomplexes, phenolic condensation products, tetraphenyl compounds, metaloxide particles, and metal oxides particles surface-treated with variouscoupling agents.

Furthermore, in order to control the charging properties of theelectrophoretic particle holder, surface-treated one can optionally beused.

As a method of surface treatment, a chemical treatment method with asurface treatment agent such as a silane-coupling agent or a physicaltreatment method in which some physical stimulus is imparted on asurface thereof to modify a surface can be exemplified. In theinvention, the chemical treatment method may be used.

As the surface treatment agents that can be used, for example, in ahydrophobic treatment, silane compounds, silicone compounds or fattyacids can be used, and in the hydrophilic treatment, alcohols,hydrophilic resins or inorganic compounds can be used.

As the silane compounds that can be used in the hydrophobic treatment, aknown silane coupling agent that has a molecular structure containing areactive portion that reacts with the electrophoretic particle bodiesand a hydrophobic portion can be used.

Specifically, octadecyltrimethoxysilane, phenetyltrimethoxysilane,aminopropyltriethoxysilane, 3-aminopropyltrimethoxysilane,methacryloxytrimethoxysilane, methoxytrimethylsilane,3-aminopropyldiethoxymethylsilane,N-(2-aminoethyl)-3-aminopropyltrimethoxysilane andN-(2-aminoethyl)-3-aminopropylmethyldimethoxysilane can be exemplified.

As the silicone compounds that are used in the hydrophobic treatment,methylpolysiloxane, octamethylcyclotetrasiloxane,decamethylcyclopentanesiloxane, methylcyclopolysiloxane andmethylhydrogenpolysiloxane can be exemplified.

As the fatty acids that are used in the hydrophobic treatment, lauricacid, myristic acid, stearic acid, oleic acid, linoleic acid, linolenicacid, hydroxy fatty acid, caproic acid, caprylic acid, palmitic acid,behenic acid, palmitoleic acid, erucic acid, alkali metal salts such assodium salts and potassium salts thereof, alkali earth metal salts suchas magnesium salts and calcium salts thereof or esters thereof can beexemplified.

As alcohols that are used in the hydrophilic treatment, methyl alcohol,ethyl alcohol, propanol, isopropanol, butyl alcohol, glycerin, propyleneglycol and 1,3-butylene glycol can be exemplified.

As the hydrophilic resins that are used in the hydrophilic treatment,acrylic acid, polyvinyl alcohol, polyvinyl pyrrolidone, polyamide andpolyimide can be exemplified.

As the inorganic oxides that are used in the hydrophilic treatment,silica, alumina and titania can be exemplified.

When the electrophoretic particle includes a soft magnetic material or aferromagnetic material, the electrophoretic particle holder may containa magnetic material. Thereby, the adherence working between theelectrophoretic particle and the electrophoretic particle holder can becontrolled by, in addition to an electrostatic force, a magnetic forceas well.

Here, when the electrophoretic particle contains a soft magneticmaterial, in the electrophoretic particle holder, a ferromagneticmaterial can be added, and, when the electrophoretic particle contains aferromagnetic material, in the electrophoretic particle holder, a softmagnetic material or a ferromagnetic material can be added. As themagnetic materials that can be used in the electrophoretic particle orthe electrophoretic particle holder, known ones can be used. That is, asthe soft magnetic materials, for example, silicon steel, Permalloy, andamorphous metal can be used, and, as the ferromagnetic materials, ironoxide, carbon steel, ferrite and samarium can be used. The kind and anaddition amount of the magnetic material that is used in theelectrophoretic particle or the electrophoretic particle holder can beselected so that a desired threshold electric field value may beobtained. Furthermore, as colored magnetic powder that can be used asthe electrophoretic particle, for example, fine particle-size coloredmagnetic powders described in, for example, JP-A No. 2003-131420 can beused.

—Electrophoretic Particle—

The electrophoretic particle that is used in the invention is a particlehaving the characteristics capable of being charged with a positive ornegative polarity so that it may move in a dispersion medium along adirection of an electric field gradient when the particle is disposed inan electric field. Examples thereof include glass beads, particles ofmetal oxide such as alumina or titanium oxide, thermoplastic orthermosetting resin particles, ones obtained by fixing a coloringmaterial on a surface of the resin particles, particles containing acoloring material in thermoplastic or thermosetting resin and particleshaving the characteristics of forming a color in a state of beingdispersed in a dispersion medium.

Examples of the thermoplastic resins that are used to produce theelectrophoretic particles include homopolymers or copolymers of styrenessuch as styrene and chlorostyrene; monoolefins such as ethylene,propylene, butylene and isoprene; vinyl esters such as vinyl acetate,vinyl propionate, vinyl benzoate and vinyl lactate; (x-methylenealiphatic monocarboxylic acid esters such as methyl acrylate, ethylacrylate, butyl acrylate, dodecyl acrylate, octyl acrylate, phenylacrylate, methyl methacrylate, ethyl methacrylate, butyl acrylate anddodecyl acrylate; vinyl ethers such as vinyl methyl ether, vinyl ethylether and vinyl butyl ether; and vinyl ketones such as vinyl methylketone, vinyl hexyl ketone and vinyl isopropenyl ketone.

Examples of the thermosetting resins that can be used to produce theelectrophoretic particles include crosslinked resins such as crosslinkedcopolymers mainly made of divinyl benzene and crosslinked polymethylmethacrylate; a phenol resin; a urea resin; a melamine resin; apolyester resin; and a silicone resin. Examples of particularly typicalbinding resins include polystyrene, a styrene-alkyl acrylate copolymer,a styrene-alkyl methacrylate copolymer, a styrene-acrylonitrilecopolymer, a styrene-butadiene copolymer, a styrene-maleic anhydridecopolymer, polyethylene, polypropylene, polyester, polyurethane, anepoxy resin, a silicone resin, polyamide, modified rosin and paraffinwax.

As the coloring materials, organic or inorganic pigments and oil-solubledyes can be used. Examples thereof include magnetic powders such asmagnetite and ferrite, carbon black, titanium oxide, magnesium oxide,zinc oxide and known coloring materials such as phthalocyanine coppercyan color material, azo yellow color material, azo magenta colormaterial, quinacridone magenta color material, red color material, greencolor material and blue color material. Specifically, aniline blue,calcoil blue, chrome yellow, ultramarine blue, Dupont oil red, quinolineyellow, methylene blue chloride, phthalocyanine blue, malachite greenoxalate, lamp black, rose Bengal, C.I. pigment red 48:1, C.I. pigmentred 122, C.I. pigment red 57:1, C.I. pigment yellow 97, C.I. pigmentblue 15:1 and C.I. pigment blue 15:3 can be exemplified as typical ones.

Furthermore, porous spongy particles incorporating air or hollowparticles can be used as white particles.

In a resin for electrophoretic particles, a charge control agent mayoptionally be added. As the charge control agent, known ones that areused as a toner material for electrophotography can be used. Examplesthereof include quaternary ammonium salts such as cetylpyridyl chloride,BONTRON P-51, BONTRON P-53, BONTRON E-84 and BONTRON E-81 (trade name,all manufactured by Orient Chemical Industries, Ltd.), salicyclic acidtype metal complexes, phenolic condensation products, tetraphenylcompounds, metal oxide fine particles, and metal oxide fine particlessurface-treated with various kinds of coupling agents.

Inside or on a surface of electrophoretic particles, a magnetic materialmay optionally be mixed. As the magnetic material, a color coatedinorganic magnetic material or organic magnetic material may be used.Furthermore, a transparent magnetic material, in particular, atransparent organic magnetic material does not disturb coloring of acoloring pigment and is smaller in the specific gravity than aninorganic magnetic material.

As the colored magnetic powder, for example, a small diameter coloredmagnetic powder described in JP-A No. 2003-131420 can be used. One thathas a magnetic particle that is a nucleus and a coloring layer laminatedon a surface of the magnetic particle is used. As the coloring layer, apigment may be selected to color a magnetic powder opaque. However, forexample, an optical interference film may be used. The opticalinterference film is a thin film that is made of an achromatic materialsuch as SiO₂ and TiO₂ and has a thickness equivalent to a lightwavelength, which wavelength-selectively reflects light due to opticalinterference in a thin film.

On a surface of the electrophoretic particle, an external additive mayoptionally be adhered. A color of the external additive may betransparent so as not to affect on a color of the particle.

As the external additive, inorganic particle of metal oxide such assilicon oxide (silica), titanium oxide or alumina is used. In order tocontrol the charging properties, fluidity and environmental dependencyof the inorganic particle, these can be surface-treated with a couplingagent or silicone oil.

As the coupling agents, there are positively charging coupling agentssuch as an aminosilane coupling agent, aminotitanium coupling agent andnitrile coupling agent and negatively charging coupling agents such as asilane coupling agent that does not contain a nitrogen atom (composed ofatoms other than nitrogen), titanium coupling agent, epoxy silanecoupling agent and acryl silane coupling agent. Furthermore, as thesilicone oils, there are positively charging silicone oils such as aminomodified silicone oils and negatively charging silicone oils such asdimethyl silicone oil, alkyl modified silicone oil, α-methylsulfonemodified silicone oil, methylphenyl silicone oil, chrolophenyl siliconeoil and fluorine modified silicone oil. These are selected according tothe desired resistance of the external additive.

Among such external additives, well-known hydrophobic silica andhydrophobic titanium oxide may particularly be used. For example, atitanium compound that is described in JP-A No. 10-3177 and obtained bya reaction between TiO(OH)₂ and a silane compound such as a silanecoupling agent may be used. As the silane compound, any one ofchrolosilane, alkoxysilane, silazane and special silyl agent can beused. The titanium compound is prepared by reacting a silane compound orsilicone oil with TiO(OH)₂ prepared in a wet process followed by drying.Since the titanium compound is not subjected to a sintering process atseveral hundred degrees centigrade, Ti and Ti will not be stronglycoupled, aggregation will not occur at all, and the particles areapproximately in a primary particle state. Furthermore, since the silanecompound or silicone oil is directly reacted with TiO(OH)₂, a processingamount of the silane compound or the silicone oil can be increased. Byadjusting the processing amount of the silane compound, the chargingcharacteristics can be controlled, and the resulting charging abilitycan be improved more significantly than that of existing titanium oxide.

The primary particle diameter of the external additive may be generallyin the range of 5 to 100 nm, and specifically in the range of 10 to 50nm without restricting thereto.

A blending ratio of the external additive and the electrophoreticparticle is adjusted appropriately according to the particle diameter ofthe electrophoretic particles and the particle diameter of the externaladditive. When the added amount of the external additive is too much,the external additive is partially separated from the surface of theparticle and sticked to the surface of the other particles, and thusdesired charging characteristics cannot be obtained. In general, anamount of the external additive is in the range of 0.01 to 3 parts byweight and more particularly in the range of 0.05 to 1 part by weightwith respect to 100 parts by weight of the particles.

When the external additive is added on a surface of the electrophoreticparticle, the external additive may be fixed on a surface of theelectrophoretic particle with impact force, or, by heating a surface ofthe electrophoretic particle, the external additive may be solidly fixedon a particle surface. Thereby, the external additive is prevented fromseparating from the electrophoretic particle, the external additives ofdifferent polarities are prevented from strongly aggregating, and anaggregate of the external additive difficult to be dissociated by anelectric field is prevented from forming, resulting in preventing imagequality deterioration.

As a method of preparing the electrophoretic particle, anyconventionally known methods can be used. For example, a methoddescribed in JP-A No. 7-325434 can be used. That is, a method where aresin, a pigment and a charge control agent are measured so as to be apredetermined mixing ratio, the resin is heated and melted, the pigmentis added thereto, followed by mixing, dispersing and cooling, furtherfollowed by preparing particle by use of a crusher such as a jet mill,hammer mill or turbo-mill, still further followed by dispersing theobtained particle in a dispersion medium. Furthermore, a method whereparticles in which a charge control agent is contained are preparedaccording to a polymerization method such as a suspensionpolymerization, emulsion polymerization or dispersion polymerization ora coacervation, melt dispersion or emulsion coagulation method, followedby dispersing in a dispersion medium to prepare a particle-dispersedliquid may be used. Still further, a method where at a temperature thatcan plasticize a resin, however does not boil a dispersion medium and islower than a decomposition temperature of the resin, charge controlagent and/or coloring agent, an appropriate apparatus that can disperseand knead the resin, coloring material, charge control agent and thedispersion medium is used can be exemplified. Specifically, a pigment,the resin and the charge control agent are heated and melted by use of aplanetary mixer or kneader in a dispersion medium, and, by using thetemperature dependency of the solubility of the resin in a solvent,cooled while a melt mixture is stirred to coagulate/precipitate toprepare electrophoretic particles.

Furthermore, a method where the above-described raw materials are placedin an appropriate vessel provided with particulate media for dispersionand kneading such as an attritor or a heated vibration mill such as aheated ball mill and the vessel is heated to an appropriate temperaturerange of for example 80 to 160° C. to disperse and knead can be used. Asthe particulate media, steels such as stainless steel and carbon steel,alumina, zirconia or silica may be used. When an electrophoreticparticle is prepared according to the method, after previously fluidizedraw materials are further dispersed in the vessel by use of theparticulate media, the dispersion medium is cooled to precipitate aresin containing the coloring agent from the dispersion medium. Theparticulate media, while a moving state is still maintained during andafter cooling, generates shear and/or impact to make a particle diametersmaller.

In the next place, electrophoretic particle that has the characteristicsof forming a color in a dispersed state in a dispersion medium will bedescribed.

Here, “forming a color in a dispersed state in a dispersion medium”means that when, in a state where electrophoretic particles aredispersed in a dispersion medium, a dispersion liquid in whichelectrophoretic particles are dispersed is visually observed, anobservable hue is exhibited. The hue at this time is observed with athickness of a dispersion liquid in an observing direction of in therange of substantially 10 μm to 1 cm. The hue can be variously changedby changing the shape and particle diameter of the electrophoreticparticles or a material that constitutes the electrophoretic particles.

When a dispersion medium positioned between a pair of substrates ispartitioned into a plurality of cells by use of partition walls, thepolarity of the electrophoretic particle may be same at least in a cellunit.

As the electrophoretic particle that forms a color in a dispersed state,coloring agents such as organic pigments, inorganic pigments, coloredglass and dyes, resin particles containing these coloring agents ormetal particles can be used, and optionally, ones obtained bysurface-treating a surface of the particles with a silane coupling agentcan be used. As one example, particles constituted of a blackcarbon-dispersed PMMA (polymethyl methacrylate) resin prepared by asuspension polymerization method can be used.

As the organic pigments, inorganic pigments and dyes that can be used asa coloring agent, known ones can be used. Examples of the organicpigments include azo pigments, polycondensation azo pigments, metalcomplex azo pigments, flavanthrone pigments, benzimidazolone pigments,phthalocyanine pigments, quinacridone pigments, anthraquinone pigments,anthrapyridine pigments, pyranthrone pigments, dioxazine pigments,perylene pigments, perinone pigments, isoindolinone pigments,quinophthalone pigments, thioindigo pigments, and indanthrene pigments.Examples of inorganic pigments include zinc oxide, titanium dioxide,zirconium oxide, antimony white, carbon black, black iron oxide,titanium boride, red iron oxide, Mapico yellow, red lead, cadmiumyellow, zinc sulfide, lithopone, barium sulfide, cadmium selenide,barium sulfate, lead chromate, lead sulfate, barium carbonate, calciumcarbonate, white lead, and alumina white. Examples of dyes includenigrosine dyes, phthalocyanine dyes, azo dyes, anthraquinone dyes,quinophthalone dyes and methane dyes.

As the resin particle including a coloring material, ones obtainedaccording to a known dry method where a resin solid matter in which acoloring material is dispersed is kneaded and pulverized or onesobtained according to a known wet method where resin particles areobtained by granulating in a dispersion liquid in which raw materialssuch as a coloring material and a resin are dispersed can be used.

As the electrophoretic particles that form a color in a dispersionstate, metal particles can be used as well, and optionally, onesobtained by surface treating a surface of the particle with a silanecoupling agent can be used as well. The metal particle may be a metalparticle containing a precious metal.

The metal particles that can be used as the electrophoretic particleshave a color strength due to surface plasmon resonance, that is, theparticles themselves have characteristics to form a color.

The color due to surface plasmon resonance of the metal particles iscaused by the plasma oscillation of the electrons, being based on thecolor formation mechanism called the plasmon absorption. It is said thatthe color formation based on the plasmon absorption is caused by thefree electrons quivered by a photoelectric field in the metal, whichresults in formation of electric charges on the particle surface togenerate a non-linear polarization. This color formation by the metalparticle is high in color saturation and light transmittance andexcellent in durability. Such a color formation by the metal particlecan be found in so-called nano-particles, which have a particle diameterin the range of substantially several to several tens nanometers. Fromviewpoints of clearer color hue, the narrower a particle sizedistribution of metal particles is, the larger an advantage is.Accordingly, an average particle diameter (volume average particlediameter) of the metal particles may be in the range of 1 to 100 nm andparticularly in the range of 5 to 50 nm.

The metal particles can form various colors depending on the kind ofmetals contained in the particles, a shape of the particle and a volumeaverage particle diameter. Accordingly, when metal particles whereinthese are controlled are used, various colors including RGB colors maybe obtained. Accordingly, when a display medium is prepared with adispersion liquid obtained by dispersing metal particles having a colorstrength due to surface plasmon resonance in a dispersion medium, acolor display can be performed. Further, when metal particle dispersionliquids of the respective colors corresponding to R, G, B are used, adisplay medium according to an RGB system can be prepared.

Volume average particle diameters of the metal particles for exhibitingthe respective colors of R, G, B in an RGB system, being dependent onmetals used, preparation conditions of the particles and shapes, cannotbe particularly restricted. However, in the case of for example goldcolloid particles, as the volume average particle diameter becomeslarger, R color formation, G color formation and B color formation tendto occur in this order.

As a method of measuring the volume average particle diameter in theinvention, a laser diffraction/scattering method where a laser beam isilluminated to a particle group, and, from intensity distributionpatterns of diffracted and scattered light emitted therefrom, an averageparticle diameter is measured is adopted. For example, a particlediameter can be measured with a MICROTRAC particle size distributionanalyzer MT3300 (manufactured by Nikkiso Co., Ltd.).

As the metals contained in the metal particles, known precious metalssuch as gold, silver, ruthenium, rhodium, palladium, osmium, iridium andplatinum may be used, and particularly gold and/or silver can be used.Metals (for example, copper) other than precious metals can be used aswell. Furthermore, two or more kinds of metals may be contained in themetal particles.

Furthermore, in order to control the charging properties of theelectrophoretic particle, a surface of the electrophoretic particle mayoptionally be surface treated (hydrophilic treatment or hydrophobictreatment).

As a method of surface treatment, a chemical treatment method with asurface treatment agent such as a silane-coupling agent or a physicaltreatment method in which some physical stimulus is imparted on asurface of the electrophoretic particles to modify a surface can beexemplified. In the invention, the chemical treatment method may beused.

A surface treatment agent that can be used can be selected in view ofthe affinity with a material that constitutes a particle body of theelectrophoretic particle. For example, in a hydrophobic treatment,silane compounds, silicone compounds or fatty acids can be used.

As the silane compounds that can be used in the hydrophobic treatment,known silane coupling agents that has a molecular structure containing areactive portion that reacts with the electrophoretic particle and ahydrophobic portion can be used.

Specifically, octadecyltrimethoxysilane, phenetyltrimethoxysilane,aminopropyltriethoxysilane, 3-aminopropyltrimethoxysilane,methacryloxytrimethoxysilane, methoxytrimethylsilane,3-aminopropyldiethoxymethylsilane,N-(2-aminoethyl)-3-aminopropyltrimethoxysilane andN-(2-aminoethyl)-3-aminopropylmethyldimethoxysilane can be exemplified.

As the silicone compounds that are used in the hydrophobic treatment,methylpolysiloxane, octamethylcyclotetrasiloxane,decamethylcyclopentanesiloxane, methylcyclopolysiloxane andmethylhydrogenpolysiloxane can be exemplified.

As the fatty acids that are used in the hydrophobic treatment, lauricacid, myristic acid, stearic acid, oleic acid, linoleic acid, linolenicacid, hydroxy fatty acid, caproic acid, caprylic acid, palmitic acid,behenic acid, palmitoleic acid, erucic acid, alkali metal salts such assodium salts and potassium salts thereof, alkali earth metal salts suchas magnesium salts and calcium salts thereof or esters thereof can beexemplified.

Furthermore, in the case where the electrophoretic particle holder has asize or own weight larger than that of the electrophoretic particle,when, other than the electrostatic force, a particle diameter of theelectrophoretic particle is controlled, an intermolecular force workingwith the electrophoretic particle holder can be controlled and therebythe adherence between both can be controlled.

“The electrophoretic particle holder has a size or own weight largerthan that of the electrophoretic particle” means that a mass of theelectrophoretic particle holder is 100 times or more of a mass of theelectrophoretic particle, and, when the “electrophoretic particleholder” is composed of electrophoretic particles, means that an averageparticle diameter thereof is 10 times or more of an average particlediameter of the electrophoretic particles.

—Dispersion Medium—

A dispersion medium contains at least an insulating liquid and a volumeresistance value thereof may be 10³ Ωcm or more, particularly in therange of 10⁷ to 10¹⁹ Ωcm and more particularly in the range of 10¹⁰ to10¹⁹ Ωcm. When the volume resistance value is set in the range, moreeffectively, air bubbles due to electrolysis of the dispersion mediumcaused by an electrode reaction can be inhibited from occurring. In thiscase, the electrophoresis characteristics of the electrophoreticparticles are not damaged every energization and excellent repetitionstability can be imparted. Furthermore, in the dispersion medium, otherthan an insulating liquid having the volume resistance value,optionally, a dispersion stabilizer such as acid, alkali, salt andsurfactant can be added, and, a stabilizer with intention of inhibitingoxidization and absorbing UV-light, an antibacterial agent and anantiseptic agent can be added. However, these may be added so that thevolume resistance value may be in the above range. The viscosity of thedispersion medium, though not particularly restricted, may be in therange of 1 to 100 mPa·s.

As liquids that can be used as the dispersion medium, knownwater-soluble organic solvents or hydrophobic organic solvents can beused. Examples thereof include hexane, cyclohexane, toluene, xylene,decane, hexadecane, kerosene, paraffin, isoparaffin, silicone oil,modified silicone oil, fluorooil, dichloroethylene, trichloroethylene,perchloroethylene, high-purity petroleum, ethylene glycol, alcohols,ethers, esters, dimethylformamide, dimethylacetamide, dimethylsulfoxide, N-methylpyrrolidone, 2-pyrrolidone, N-methylfornamide,acetonitrile, tetrahydrofuran, propylene carbonate, ethylene carbonate,benzine, diisopropylnaphthalene, olive oil, isopropanol,trichlorotrifluoroethane, tetrachloroethane, dibromotetrafluoroethaneand the mixtures thereof.

Furthermore, when removing impurities so as to be in the above-mentionedvolume resistance value, water (so-called pure water) may be used aswell.

Among the liquids exemplified above, hydrophobic organic solvents suchas hexane, cyclohexane, kerosene, paraffin and silicone oil, which aredecomposed with difficulty even when a voltage is applied, may be used,and particularly silicone oil may be used.

The silicone oil, in comparison with the dispersion medium such ashexane, cyclohexane, kerosene and paraffin, which are so far used in aconventional display medium of an electrophoresis system, has thecharacteristics in that (1) a dispersion medium is decomposed withdifficulty even when a higher voltage is applied, (2) due to highviscosity, when metal particles are electrophoresed, violent convectionoccurs with difficulty; accordingly, deterioration of the contrast andthe disturbance of the display due to violent convection are caused withdifficulty, and (3) at the preparation of a display medium, when adispersion liquid in which metal particles are dispersed is filled underreduced pressure in a space in the display medium where a dispersionmedium is to be positioned, the dispersion medium is volatilized withdifficulty.

As the silicone oil, known silicone oils can be used without particularrestriction. (1) The resistance value may be 10³ Ωcm or more,particularly in the range of 10⁷ to 10¹⁹ Ωcm and more particularly inthe range of 10¹⁰ to 10¹⁹ Ωcm. (2) The viscosity may be in the range of1 to 1000 cst and particularly in the range of 1 to 100 cst.Specifically, dimethyl silicone oils such as KF-96 (trade name,manufactured by Shin-Etsu Chemical Co., Ltd.), DOW CORNING 200 (tradename, manufactured by Dow Corning Co.,) and TSF451 (trade name,manufactured by GE/Toshiba Silicone Co., Ltd.) can be used. Furthermore,modified silicone oil (for example KF-393 and X22-3710: trade name,manufactured by Shin-Etsu Silicone Co., Ltd.) where in a part of methylgroups of dimetylpolysiloxane, an organic group is introduced can beused.

—Substrate and Electrode—

As the substrate that is used in a display medium of the exemplaryembodiment, films or sheet substrates of polymers such as polyester (forexample, polyethylene terephthalate), polyimide, polymethylmethacrylate, polystyrene, polypropylene, polyethylene, polyamide,nylon, polyvinyl chloride, polyvinylidene chloride, polycarbonate,polyether sulfone, silicone resin, polyacetal resin, fluororesin,cellulose derivative and polyolefin and inorganic substrates such asglass substrate, metal substrate or ceramic substrate may be used.

At least one of a pair of substrates that are used in a display mediumis a substrate having optical transparency to visible light (transparentsubstrate). When a light-transmissive display medium is prepared, forboth substrates, transparent substrates are used. However, the displaymedium of the exemplary embodiment fundamentally may be a reflectivetype. In this case, as one substrate (back substrate) that is disposedfacing a transparent substrate, a substrate that does not have opticaltransparency is used. In a description below, unless statedparticularly, a reflective type display medium is presumed anddescribed. However, a display medium of the invention is not restrictedonly to the reflective type.

In the invention, “having optical transparency” means that the lighttransmittance to light having a wavelength of a visible region is atleast 50% or more, and the light transmittance may be specifically 80%or more and more specifically 100%.

As the transparent substrate, a glass substrate or a transparent resinsubstrate constituted of an acrylic resin, a polycarbonate resin or apolyethylene terephthalate resin may be used, and a combination thereofcan be used as well. Furthermore, as the back substrate, one having samematerial as that of the transparent substrate may be used. However, anopaque or colored substrate as well can be used. For example, a resinsubstrate constituted of an ABS resin (acrylonitrile/butadiene/styreneresin) or a glass/epoxy resin can be used.

Furthermore, the substrate may optionally be provided with an electrode.For example, when an electrode is disposed on a surface at alight-modulating layer side of a transparent substrate, an electrode(transparent electrode) that has optical transparency and is constitutedof ITO (Indium Tin Oxide) can be used. Still further, when an electrodeis disposed on a surface at a light-modulating layer side of the backsubstrate, a transparent electrode constituted of a transparentconductive material such as ITO may be used. However, an electrodeconstituted of a metal such as copper may be disposed.

The electrodes may be disposed in so-called row and column so that anelectrode at a transparent substrate side and an electrode at a backsubstrate side may intersect orthogonally by disposing in strips on asubstrate surface. Furthermore, when an electrode is disposed on asurface at a light-modulating layer side of the back substrate, in orderto protect the electrode, an insulating film constituted of a resin oran inorganic material may be disposed so as to cover the electrodesurface.

On the substrate, a wiring, a thin film transistor, a diode having ametal/insulating layer/metal structure, a variable capacitor and adriving switch element such as a ferroelectric material may optionallybe formed.

—Other Materials—

In a display medium, in order to inhibit contents such as a dispersionmedium from leaking from the display medium or in order to partitionwith partition walls a light-modulating layer constituted of adispersion medium and the like positioned between a pair of substratesinto a plurality of cells, a partition wall is disposed between a pairof substrates.

A height of the partition wall, without particularly restricted, isusually in the range of substantially 20 μm to 1 mm. Furthermore, awidth of the partition wall, though not particularly restricted, isadvantageously smaller from the viewpoint of the resolution of thedisplay medium, and usually in the range of substantially 10 μm to 1 mm.

Furthermore, a material of the partition wall, as far as it is amaterial having an insulating property and insoluble in a dispersionmedium, is not particularly restricted. For example, a knownphotosensitive resin or rubber can be used.

In the exemplary embodiment, the “insulating property” means that thevolume resistivity is 10⁶ Ωcm or more. Furthermore, a “conductiveproperty” means that the volume resistivity is 10⁻³ Ωcm or less.

In addition, at the time of preparing a display medium, in order toadhere a partition wall and a substrate, an adhesive can be used. Theadhesive is not particularly restricted. A thermosetting resin or aUV-curable resin can be used. However, a material that does not affecton a material of the partition wall or a material that constitutes alight-modulating layer is selected.

Furthermore, in order to maintain a gap width between a pair ofsubstrates constant, optionally, a rib may be disposed or particleshaving a size same as the gap width of the pair of substrates may bedisposed.

—Method of Producing Display Medium—

A method of producing a display medium is not particularly restricted.However, it may be produced according to, for example, a process below.In the beginning, as a pair of substrates, a transparent substrate and aback substrate are prepared. The substrates may be previously providedwith an electrode. Subsequently, partition walls are formed on a surfaceat a light-modulating layer side of any one of the transparent substrateand the back substrate, followed by adhering both substrates to eachother. When the substrates are adhered, electrophoretic particle holdersuch as electrophoretic particle holding particles are filled between apair of substrates. In the next place, from an injection port of adispersion medium, which is provided in advance at the time of formingpartition walls, a dispersion medium containing one or more kinds ofelectrophoretic particle are injected, followed by sealing the injectionport to obtain a display medium.

—Display Device—

In the next place, a display device that uses a display medium of theabove-described exemplary embodiment will be described. A display deviceof the invention includes the display medium of the invention whichincludes a pair of electrodes at positions capable of applying anelectric field to a dispersion medium positioned between a pair ofsubstrates, and further includes an electric field applicator that isconnected to the pair of electrodes and applies an electric field to thedispersion medium. Here, the pair of electrodes is disposed at onesubstrate side of a pair of substrates and at the other substrate sidethereof.

For example, when on a surface at a transparent substrate side of a backsubstrate a back electrode is disposed and on a surface at an oppositeside to a display surface of a transparent substrate a transparentelectrode is disposed, an electric field applicator is connected to thepair of electrodes. Thereby, when a display is carried out, withoutusing an electric field applicator outside of the display medium, thedisplay can be carried out.

The electric field applicator that is used in the display device, as faras it is provided with a control function by which a voltage waveform ofan applied electric field and an electric field application periodthereof can be controlled so as to enable to display a halftone, is notparticularly restricted. However, usually, an electric field applicatorthat can apply an electric field having a voltage waveform thatsatisfies the formula (5) to a dispersion medium is used, and one thatcan apply an electric field having a voltage waveform that satisfies theformula (7) or (8) to a dispersion medium may be used. Accordingly, asan electric field applicator, for example, an electric field applicatorthat includes a control circuit or is provided with a program, which cancontrol so that a voltage waveform of an electric field applied maysatisfy the formula (5), (7) or (8), can be used.

Furthermore, as the electric field applicator, any one of an AC powersource and a DC power source may be used, and, when an AC voltage and aDC voltage are applied simultaneously through electrodes to thelight-modulating layer, both can be used together.

—Specific Example of Display Medium (Display Device)—

In what follows, specific examples of display devices will be describedwith reference to the drawings. However, the display medium of theexemplary embodiment is not restricted only to examples described below.

FIG. 8 is a schematic diagram showing an example of a display medium ofthe exemplary embodiment. In the drawing, reference numeral 100 denotesa display medium; reference numeral 200, a transparent substrate;reference numeral 202, a display surface; reference numeral 204, a backsubstrate; reference numeral 206, a partition wall; reference numeral210, a transparent electrode; reference numeral 220, a back electrode;reference numeral 300, an electrophoretic particle; reference numeral302, a dispersion medium; reference numeral 401, an electrophoreticparticle holder; reference numeral 400, an electrophoretic particleholding particle; and reference numeral 500, an electric fieldapplicator.

A display medium 100 shown in FIG. 8 includes: a transparent substrate200 one surface of which constitutes a display surface 202 and the othersurface of which is provided with a transparent electrode 210; a backsubstrate 204 that is disposed facing a surface of the transparentsubstrate 200 on which the transparent electrode 210 is disposed andthat is provided with a back electrode 220 on the transparent substrate200 side thereof; a partition wall 206 that is disposed at an endportion of a gap between the transparent substrate 200 and the backsubstrate 204 so as to seal the gap portion; a dispersion medium 302containing electrophoretic particles 300 positioned in a gap between thetransparent substrate 200 and the back substrate 204; andelectrophoretic particle holder 401 disposed between a pair oftransparent substrates 200 and 204.

Here, a region surrounded by a pair of transparent substrates 200 and204 and a partition wall 206 corresponds to a light-modulating layer.Furthermore, a transparent electrode 210 and a back electrode 220 of thedisplay medium 100 are connected to an electric field applicator 500 sothat an electric field may be applied to a dispersion medium 302; andfor example, an electric field having a rectangular voltage waveform(so-called pulse wave) that satisfies the formula (7) or (8) can beapplied. Furthermore, an electrophoretic particle holder 401 is composedof an aggregate of a plurality of electrophoretic particle holdingparticles 400 and filled and disposed between a pair of substrates 200and 204 to an extent where the plurality of electrophoretic particleholding particles 400 cannot move to each other.

In the next place, an example of an operation of a display medium 100will be described of a case where electrophoretic particle holdingparticles 400 are white and charged negative and electrophoreticparticles 300 are red and charged positive.

In the beginning, as shown in FIG. 8, in a case where theelectrophoretic particles 300 are localized on a surface of theelectrophoretic particle holding particles 400 present at a transparentsubstrate 200 side, at a side that faces the transparent substrate 200,when the display medium 100 is observed from a display surface 202 side,a deep red color (at the maximum density or similar density to that) isobserved.

Here, with the transparent electrode 210 set minus and the backelectrode 220 set plus, an electric field is applied for a sufficienttime so that the maximum density may be completely obtained, and a statewhere the electric field has been applied is taken as an initial state.

While the electric field is being applied, electrophoretic particles 300are attracted toward a surface of the transparent electrode 210.However, when the electric field is removed, the electrophoreticparticles 300 become readily detachable from a surface of thetransparent electrode 210. However, the electrophoretic particles 300,even after detached from a surface of the transparent electrode 210, arestuck and held on a surface (surface at a transparent electrode 210side) of the electrophoretic particle holding particle 400 neighboringto a surface of the transparent electrode 210; accordingly, the displaydensity after the electric field is removed can be stably sustained overtime.

In the next place, when, from the initial state, an electric fieldhaving a pulse voltage waveform is applied so as to satisfy, forexample, the formula (8) and so that an integer n of the formula (8) maybe 3, a four-level display of deep red (maximum density), slightly deepred, slightly thin red and white (minimum density) can be realized.

Furthermore, even when a state where an electric field is removed issustained after a gradation of any one of levels is displayed, theelectrophoretic particles 300 continue to be stuck and held at the samepositions of a surface of the electrophoretic particle holding particle400 as that immediately after the electric field is removed;accordingly, the display density does not vary over time.

In the next place, an example of a display medium where as anelectrophoretic particle holder an electrophoretic particle holderconstituted of a porous body or a network structure is used will bedescribed.

FIG. 9 is a schematic diagram showing another example of a displaymedium of the exemplary embodiment, and, in the drawing, a referencenumeral 110 denotes a display medium, a reference numeral 410 denotes,in place of the electrophoretic particle holder 401 constituted of anaggregate of the electrophoretic particle holding particles 400 shown inFIG. 8, an electrophoretic particle holder 410 constituted of anaggregate of porous bodies, network structures or fibers, and membersrepresented by other reference numerals have same functions as thatshown in FIG. 8.

The display medium 110 shown in FIG. 9 fundamentally has a configurationsimilar to the display medium 100 shown in FIG. 8. However, it isdifferent from the display medium 100 in a point that, as theelectrophoretic particle holder, an electrophoretic particle holder 410constituted of a porous body, network structures or an aggregate offibers is used. The electrophoretic particle holder 410 constituted ofporous bodies, network structures or aggregates of fibers are filled anddisposed between a pair of substrates 200 and 204 so as not to be ableto move in a light-modulating layer. As to the display medium 110 aswell, a display similar to that of the display medium 100 shown in FIG.8 can be carried out.

The electrophoretic particle holder 410 constituted of porous bodies,network structures or aggregates of fibers filled and disposed between apair of substrates 200 and 204 of a display medium 110, as shown in FIG.9, may be disposed so that a holding function of the electrophoreticparticles 300 in a direction in which the pair of substrates 200 and 204face each other may be uniform. However, the electrophoretic particleholder 410 may be disposed so that a holding function of theelectrophoretic particles 300 in a direction in which the pair ofsubstrates 200 and 204 face each other may decrease continuously orstepwise as a distance from at least one of the transparent substrate200 and the back substrate 204 increases.

When the electrophoretic particle holder 410 constituted of the porousbodies, network structures or aggregates of fibers is filled anddisposed between substrates 200 and 204 of a display medium 110, incomparison with a case where the electrophoretic particle holder isconstituted of an aggregate of the electrophoretic particle holdingparticles 400, the air permeability, aperture, inter-fiber distance andpore density can be readily adjusted. Furthermore, the electrophoreticparticle holder can be constituted as a continuous body as a continuousmember, and, thereby, the function of holding the electrophoreticparticles 300 of the electrophoretic particle holder may be partiallyadjusted.

Specifically, as shown in FIG. 21, the electrophoretic particle holder410 may be filled and disposed so that, among the electrophoreticparticle holder 410 filled and disposed between the transparentsubstrate 200 and the back substrate 204, the electrophoretic particleholder 410A disposed at a transparent substrate 200 side may have alarger holding function of the electrophoretic particles 300, and theelectrophoretic particle holder 410B disposed in a region far from thetransparent substrate 200, that is, at a back substrate 204 side mayhave a smaller holding function of the electrophoretic particles 300than that in a region close to the transparent substrate 200.

Furthermore, as shown in FIG. 22, the display medium may be constitutedin such a manner that, among the electrophoretic particle holder 410filled and disposed between the transparent substrate 200 and the backsubstrate 204, electrophoretic particle holder 410A disposed in a regionclose to the transparent substrate 200 and electrophoretic particleholder 410C disposed in a region close to the back substrate 204 mayhave a larger holding function of the electrophoretic particles 300, andelectrophoretic particle holder 410B smaller in the holding function ofthe electrophoretic particles 300 than that of the electrophoreticparticle holder 410A and the electrophoretic particle holder 410C may befilled and disposed in a region between the electrophoretic particleholder 410A and the electrophoretic particle holder 410C so that theholding function of the electrophoretic particles 300 may decrease as adistance from each of the transparent substrate 200 and the backsubstrate 204 increases in a direction where the substrates face eachother.

In order to continuously or stepwise vary the holding function of theelectrophoretic particles 300 of the electrophoretic particle holder 410in a direction where the transparent substrate 200 and the backsubstrate 204 face each other, as described above, the charging amount,air permeability, aperture, pore density and inter-fiber distance of theelectrophoretic particle holder 410 may be adjusted.

When, as shown in FIG. 22, among the electrophoretic particle holder 410filled and disposed between the transparent substrate 200 and the backsubstrate 204, the holding functions of the electrophoretic particles300 of the electrophoretic particle holder 410A constituting a regionclose to the transparent substrate 200 and the electrophoretic particleholder 410C constituting a region close to the back substrate 204,respectively, are adjusted to be larger than the holding function of theelectrophoretic particles 300 of the electrophoretic particle holder410B constituting a region between the electrophoretic particle holder410A and the electrophoretic particle holder 410C of the electrophoreticparticle holder 410, only the electrophoretic particle holder 410Bdisposed therebetween may be a colored layer. In this case, a color ofthe electrophoretic particle holder 410B may be white.

In order to make the color of the electrophoretic particle holder 410Bwhite, a white material may be fixed on a surface of a fiber or a memberthat constitutes the electrophoretic particle holder 410B. As the whitematerial, white pigments such as titanium oxide, barium titanate, bariumsulfate and calcium carbonate can be used. As the white material, aparticulate white particle can be used.

The display medium 110, display medium 110A and display medium 110B inwhich the electrophoretic particle holder 410 constituted of porousbodies, network structures or aggregates of fibers are filled anddisposed between the transparent substrate 200 and back substrate 204may be manufactured in a process wherein the electrophoretic particleholder 410 constituted of the porous bodies, network structures oraggregates of fibers are impregnated with the dispersion medium 302 andthe electrophoretic particles 300, and the resulting product issandwiched between the transparent substrate 200 and back substrate 204.Accordingly, the display medium 110, display medium 110A and displaymedium 110B can be readily prepared.

In all of the display media exemplified above, a case where one kind ofcolor forming electrophoretic particles is contained in one cellobtained by dividing a dispersion medium positioned between a pair ofsubstrates with a partition wall is shown. However, two or more kinds ofcolor forming electrophoretic particles may be contained in one cell. Aconfiguration example of this type display medium will be describedbelow.

FIG. 10 is a schematic diagram showing another example of a displaymedium of the exemplary embodiment. In the drawing, reference numerals120 denotes a display medium, reference numerals 300C, 300M and 300Ydenote electrophoretic particles and members shown with other referencenumerals have functions same as those shown in FIG. 8.

The display medium 120 shown in FIG. 10, though having a configurationfundamentally same as that of the display medium 100 shown in FIG. 8, isdifferent in that in the display medium 100 shown in FIG. 8, not onekind, but three kinds of electrophoretic particles 300C, 300M and 300Yare used.

As the three kinds of electrophoretic particles 300C, 300M and 300Y,ones that are charged with the same polarity, form colors different fromeach other when these dispersed in the dispersion medium 302, and aredifferent in absolute values of the threshold electric field valuescapable of detaching from a surface of the electrophoretic particleholder 400 and moving in the dispersion medium 302 are used.

In a description below, the electrophoretic particle 300C is constitutedof a cyan colored electrophoretic particle, the electrophoretic particle300M is constituted of a magenta colored electrophoretic particle andthe electrophoretic particle 300Y is constituted of a yellow coloredelectrophoretic particle, all of which being charged positive.

Here, the threshold electric field values of the three kinds of theelectrophoretic particles 300C, 300M and 300Y can be set as shown inFIG. 11 for example.

FIG. 11 is a graph describing relationship between the thresholdvoltages of three kinds of electrophoretic particles that are used inthe display medium shown in FIG. 10 and display densities thereof. Inthe drawing, a “voltage” shown in a horizontal axis means a voltage (V)of an electric field applied between a transparent electrode 210 and aback electrode 220 and is plus (rightward direction in the graph) when avoltage is applied so that a back electrode 220 side may be a positiveelectrode and a transparent electrode 210 side may be a negativeelectrode. The “display density” shown in a vertical axis means a colordensity (relative density) shown on a display surface 202 when the threekinds of electrophoretic particles 300C, 300M and 300Y, respectively,are assumed to be used singly, and a state where the electrophoreticparticles move toward the transparent substrate 200 and the displaydensity becomes larger means an upward direction of the graph.Furthermore, V1, V2 and V3 mean threshold voltages. Since a distancebetween the transparent electrode 210 and the back electrode 220 isconstant, the threshold value of an electric field and the thresholdvoltage are proportionate.

As apparent from FIG. 11, when the electrophoretic particle 300C istaken as an example for description, when a plus electric field isapplied to a dispersion medium 302, an intensity thereof is continuouslyincreased in a plus side, and a voltage becomes threshold voltage V1 orhigher, the electrophoretic particle 300C moves from a back substrate204 side to a transparent substrate 200 side, so that the displaydensity of a cyan color becomes deeper, and the display density of thecyan color saturates before the voltage reaches V2. When, in this state,a minus electric field is applied to the dispersion medium, an intensitythereof is continuously increased in a minus side, and a voltage becomesthreshold voltage −V1 or lower, the electrophoretic particle 300C movesfrom a transparent substrate 200 side to a back substrate 204 side, sothat the display density of a cyan color becomes thinner, and thedisplay density of the cyan color becomes minimum before the voltagereaches −V2.

Similarly to the above, in the case of the electrophoretic particle300M, when a voltage becomes a threshold voltage V2 or higher (or −V2 orlower), the display density increases (or decreases), and, before thevoltage reaches V3 (or −V3), the display density saturates (or becomesminimum). In the case of electrophoretic particle 300Y, when a voltageis a threshold voltage V3 or higher (or −V3 or lower), the displaydensity increases (or decreases), and, when the voltage reaches V3+α(or−V3−α), the display density saturates (or becomes minimum).

Furthermore, as shown in FIG. 11, absolute values of the thresholdvoltages of the three kinds of electrophoretic particles 300C, 300M and300Y satisfy relationship of |V1|<|V2|<|V3|. Accordingly, when a voltageis applied using the differences of the absolute values of the thresholdvoltages of the respective electrophoretic particles 300C, 300M and300Y, in one cell, in addition to white color (W), cyan color (C),magenta color (M), yellow color (Y) and second or third colors of CMYcan be displayed.

A method of controlling a threshold voltage is not particularlyrestricted. However, for example, a method where in order that theadherence to a white electrophoretic particle holding particle 400 ofeach of the three kinds of electrophoretic particles 300C, 300M and 300Ymay be differentiated from each other, average particle diameters of theelectrophoretic particles 300C, 300M and 300Y, a surface treatment stateof particle and a charge control agent applied to a particle surface areselected can be exemplified.

For example, in the case of materials constituting electrophoreticparticles 300C, 300M and 300Y and the charging characteristics due to asurface treatment state of particles being assumed substantially same,when average particle diameters of the three kinds of electrophoreticparticles 300C, 300M and 300Y are differentiated, the absolute values ofthe threshold voltages for each of the electrophoretic particles 300C,300M and 300Y can be set.

In this case, in order that the absolute values of the thresholdvoltages may satisfy the relationship of |V1|<|V2|<|V3|, the respectiveaverage particle diameters Dc, Dm and Dy of the electrophoreticparticles 300C, 300M and 300Y may satisfy the relationship of Dc>Dm>Dy,for example, Dc=500 nm, Dm=150 nm and Dy=50 nm.

In the invention, the threshold voltage exemplified in FIG. 11 can bereadily determined from a gap length of a pair of electrodes used toapply an electric field (a gap length between a transparent electrode210 and a back electrode 220 in an example shown in FIG. 10) and avoltage at which the display density of a particular color (cyan,magenta and yellow in an example shown in FIG. 10) starts varying when avoltage applied to the pair of electrodes are varied and a colordisplayed on a display medium is observed.

A method where, in the beginning, an electric field is stopped in astate where particles are drawn toward an electrode at one side, andthen the electric field is applied in an opposite direction to observewhether or not the particles move can be used to measure. For example,when, by use of a reflection densitometer, a display density is measuredto obtain the relationship between an applied voltage and the displaydensity, the threshold voltage can be obtained as a voltage value wherethe density begins varying.

Alternatively, an apparatus in which with a pair of parallel planarelectrodes dipped in a transparent vessel, a space between electrodescan be observed from a direction vertical to a parallel electric fieldis prepared. For example, when the particles are positively charged,with one electrode set to the negative polarity, the particles are movedtoward the electrode side. In this state, an electric field is shutdown, and with the electrode at a side where the particles are collectedset to the positive polarity, a voltage is gradually increased. At thistime, a voltage V at which the particles starts moving toward theelectrode at a negative side is obtained, followed by dividing by adistance between electrodes, whereby a threshold value of the electricfield can be obtained.

In all cases, in the case where electrophoresed particles do not have athreshold voltage, when the electric field is set 0 or a voltage is madesmaller, the electrophoretic particles collected on an electrode surfacestart moving, and, without applying an electric field of inversepolarity, instantaneously, the display density starts varying. On theother hand, in the case where the electrophoretic particles have athreshold value, only by weakening or removing the electric field, theelectrophoretic particles once collected on the electrode do not move ina liquid or toward an electrode at an opposite side, that is, thedisplay density does not vary.

In the next place, under the premises that three kinds ofelectrophoretic particles 300C, 300M and 300Y satisfy the relationshipshown in FIG. 11, an example of a display operation of a display medium120 will be described below.

FIGS. 12 through 20 are schematic diagrams showing an example of adisplay state in a display medium shown in FIG. 10. In the drawings,although an electric field applicator 500 is omitted from describing, apair of electrodes 210 and 220 are connected to the electric fieldapplicator 500 (FIG. 10).

In the beginning, when a voltage is applied so that a voltage may beV3+α until a display density is saturated, electrophoretic particles300C, 300M and 300Y move toward a transparent substrate 200 side todisplay a black color on a display surface 202 due to subtractive colormixing of yellow, magenta and cyan (FIG. 12).

Although, when an electric field is being applied, the electrophoreticparticles 300C, 300M and 300Y are drawn toward a surface of thetransparent electrode 210, when the electric field is removed, theelectrophoretic particles 300C, 300M and 300Y tend to be readilydetached from a surface of the transparent electrode 210. However, theelectrophoretic particles 300C, 300M and 300Y, even when detached from asurface of the transparent electrode 210, are stuck and held on asurface of the electrophoretic particle holding particle 400 near asurface of the transparent electrode 210 (a surface at a transparentelectrode 210 side); accordingly, a display density after the electricfield is removed is stably maintained over time.

Subsequently, when a voltage is applied so that a voltage may be −V3−αuntil a display density is saturated, the electrophoretic particles300C, 300M and 300Y move toward a back substrate 204 side. The threekinds of particles, when observed from a display surface 202 side, arehidden by the electrophoretic particle holding particle 400;accordingly, a white color is displayed on a display surface 202 (FIG.13).

While an electric field is being applied, the electrophoretic particles300C, 300M and 300Y are drawn toward a surface of the back electrode220. However, when the electric field is removed, these particles tendto be readily detached from a surface of the back electrode 220.However, the electrophoretic particles 300C, 300M and 300Y, even whendetached from a surface of the back electrode 220, are stuck and held ona surface of the electrophoretic particle holding particle 400 near asurface of the back electrode 220 (a surface at a back electrode 220side); accordingly, a display density after the electric field isremoved is stably maintained over time.

In the next place, for example, under the premise that an electric fieldhaving a pulse voltage waveform where the formula (8) is satisfied, aninteger n in the formula (8) is set at 2 and time Tp(1)=Tp(2)=Tmax/2=This applied to display, a description will be given.

In this case, with respect to each of the electrophoretic particles300C, 300M and 300Y, three-level display can be realized.

That is, with respect to each of the electrophoretic particles 300C,300M and 300Y, a state where the electrophoretic particles are localizedat one substrate side is taken as a first initial state (for example,the maximum density). When a voltage capable of electrophoresing theelectrophoretic particles from the first initial state to the othersubstrate side has been applied only for a time Th, the electrophoreticparticles are localized in the neighborhood of a center portion in athickness direction of a light-modulating layer to form an intermediatedensity, and then, when a voltage capable of electrophoresing theelectrophoretic particles toward the other substrate side has beenapplied only for a time Th, a second initial state (for example, theminimum density) is obtained. Furthermore, when a voltage capable ofelectrophoresing the electrophoretic particles from the first initialstate to the other substrate side has been applied for a time 2Th(Tmax), a second initial state is obtained.

Here, when from a display state (white display) shown in FIG. 13, avoltage equal to or higher than V2 but less than V3 is applied for atime 2Th (or longer), the electrophoretic particles 300C and 300M movetoward the transparent substrate 200 side. At this time, since, withrespect to the electrophoretic particles 300Y, the adherence force withthe electrophoretic particle holding particle 400 is larger than a forcereceived from the electric field, the electrophoretic particles 300Yremain stuck and held on a surface of the electrophoretic particleholding particle 400. Accordingly, a blue color is displayed on adisplay surface 202 (FIG. 14).

In the next place, when, from a display state shown in FIG. 14, avoltage equal to or less than −V1 but exceeding −V2 is applied only fora time 2Th (or longer), only the electrophoretic particles 300C aredetached from a surface of the electrophoretic particle holding particle400 and move toward the back substrate 204 side. However, since theadherence force with the electrophoretic particle holding particle 400is larger than a force receiving from an electric field, theelectrophoretic particles 300M and 300Y are still held and stuck on asurface of the electrophoretic particle holding particle 400.Accordingly, only the electrophoretic particles 300M are localized atthe transparent substrate 200 side, and a magenta color is displayed ona display surface 202 (FIG. 15).

Furthermore, when, from a display state (white display) shown in FIG.13, a voltage equal to or higher than V2 but less than V3 is appliedonly for a time Th, the electrophoretic particles 300C and 300M movefrom the back substrate 204 side to the transparent substrate 200 side,and at a time point when the electric field has been applied, theelectrophoretic particles 300C and 300M are stuck and held on thesurface of the electrophoretic particle holding particle 400 so as to belocalized in the neighborhood of a center portion in a thicknessdirection of a light-modulating layer. Accordingly, a pale blue color isdisplayed on a display surface 202 (FIG. 16).

In the next place, when, from a display state shown in FIG. 16, avoltage equal to or less than −V1 but exceeding −V2 is applied only fora time Th, only the electrophoretic particles 300C are detached from asurface of the electrophoretic particle holding particle 400 and movetoward the back substrate 204 side, and only the electrophoreticparticles 300M continue to be held as it is on a surface of theelectrophoretic particle holding particle 400 so as to be localized inthe neighborhood of a center portion in a thickness direction of alight-modulating layer. Accordingly, a thin magenta color is displayedon a display surface 202 (FIG. 17).

Furthermore, when, from a display state (black display) shown in FIG.12, a voltage equal to or lower than −V2 but exceeding −V3 is appliedonly for a time 2Th (or longer), the electrophoretic particles 300C and300M move from the transparent substrate 200 side to the back substrate204 side, and, at a time point when an electric field has been applied,the electrophoretic particles 300C and 300M are localized at the backsubstrate 204 side and only the electrophoretic particles 300Y continueto be localized at the transparent substrate 200 side similarly tobefore the application of an electric field. Accordingly, a yellow coloris displayed on a display surface 202 (FIG. 18).

In the next place, when, from a display state shown in FIG. 18, avoltage equal to or higher than V1 but less than V2 is applied only fora time 2Th (or longer), the electrophoretic particles 300C move from aback substrate 204 side to a transparent substrate 200 side, and, at atime point when an electric field has been applied, the electrophoreticparticles 300C are localized at the transparent substrate 200 side.

Accordingly, a green color is displayed on a display surface 202 due tothe subtractive color mixing of yellow and cyan (FIG. 19).

Furthermore, when, from a display state shown in FIG. 19, a voltageequal to or higher than V1 but less than V2 is applied only for a timeTh, the electrophoretic particles 300C move from the transparentsubstrate 200 side to the back substrate 204 side, and, at a time pointwhen an electric field has been applied, are stuck and held on a surfaceof the electrophoretic particle holding particle 400 so as to belocalized in the neighborhood of a center portion in a thicknessdirection of a light-modulating layer. Accordingly, a color density of acyan color becomes thinner than a display state (green color) shown inFIG. 19 to display a yellow green color on a display surface 202 (FIG.20).

In all display states shown in FIGS. 12 through 20, as far as anelectric field is not applied newly, the electrophoretic particles 300C,300M and 300Y continue to be stuck and held at same positions on thesurface of the electrophoretic particle holding particle 400;accordingly, a display density after an electric field is removed isstably maintained over time.

In the same manner as in FIGS. 10 through 20, in the cases of a displaymedium 110, a display medium 110A and a display medium 110B shown inFIGS. 9, 21 and 22, a multi-color display can be carried out by use ofplural kinds of electrophoretic particles 300C, 300M and 300Y.

The electrophoretic particle holder 410 filled and disposed inside ofthe display medium 110A and display medium 110B shown in FIGS. 21 and 22are filled and disposed so that, as a distance from at least one of thesubstrate 202 and back substrate 204 increases in a direction in whichsubstrates face each other, a function of holding the electrophoreticparticles 300 becomes smaller; accordingly, when the same kind of theelectrophoretic particles move between the substrate 202 and the backsubstrate 204, a moving speed when the electrophoretic particles move ina region larger in a function of holding the electrophoretic particles300 of the electrophoretic particle holder 410 (electrophoretic particleholder 410A and electrophoretic particle holder 410C) becomes slowerthan a moving speed when the electrophoretic particles move in a region(electrophoretic particle holder 410B) smaller in the function ofholding the electrophoretic particles 300 than the above region.

That is, when the electrophoretic particle holder 410 are filled anddisposed so that, as a distance from at least one of the substrate 202and the back substrate 204 increases in a direction in which thesubstrates face each other, the function of holding electrophoreticparticles 300 may be smaller, the moving speed of the electrophoreticparticles 300 in a region near a center between the substrate 202 andthe back substrate 204 can be made higher than the moving speed of theelectrophoretic particles 300 in a region close to at least one of thesubstrate 202 and the back substrate 204. Accordingly, at least in oneof the substrate 202 and the back substrate 204, the electrophoreticparticles 300 can be favorably held. Furthermore, when a function ofholding the electrophoretic particles 300 on the electrophoreticparticle holder 410 is varied in a direction in which the substrate 202and the back substrate 204 face each other, gradation expression can bereadily carried out.

EXAMPLES

In what follows, the exemplary embodiments will be described withreference to examples. However, the invention is not restricted toexamples shown below.

<Preparation of Electrophoretic Particle A>

A mixture constituted of 90 parts by weight of styrene monomer and 1part by weight of azoisobutyl nitrile is subjected to ball millpulverization for 20 hr by use of zirconia ball of 10 mm φ, and thereby,a dispersion liquid A-1 is obtained.

A mixture constituted of 30 parts by weight of calcium carbonate and 70parts by weight of water is subjected to an operation similar to thedispersion liquid A-1, and thereby, a dispersion liquid A-2 is obtained.

After 18 parts by weight of the dispersion liquid A-2 and 50 parts byweight of an aqueous solution of 20% sodium chloride are stirred andmixed, 30 parts by weight of the dispersion liquid A-1 is added toemulsify, and, thereby, an emulsion liquid A-3 is obtained.

The obtained emulsion liquid A-3 is heated at 70° C. under nitrogen gasflow, followed by stirring for 20 hr, and a solid particle A-4 isobtained. To the obtained solid particle A-4, 15 parts by weight of 35%hydrochloric acid is added and stirred to dissolve calcium carbonate,thereafter suction filtering and water washing are repeated 5 times, anda transparent particle A-5 is obtained.

The obtained transparent particle A-5 is mixed with methyl hydrogensilicone oil (trade name: KF-99, manufactured by Shin-Etsu Chemical Co.,Ltd.), followed by stirring to apply hydrophobic treatment, thereby anelectrophoretic particle A positively charged in silicone oil isobtained. An average particle diameter of the electrophoretic particlesA is 1 μm.

<Preparation of Electrophoretic Particle B>

A mixture of 40 parts by weight of an ethylene-methacrylic acidcopolymer (trade name: NEWCREL N699, manufactured by DUPONT Corp.,copolymerization ratio (molar ratio) of ethylene/methacrylicacid=89/11), 8 parts by weight of a magenta pigment (trade name: CARMINE6B, manufactured by Dainichiseika Color & Chemicals, Incorporated) and 2parts by weight of a charge control agent (trade name: COPY CHARGE PSYVP2038, manufactured by Clariant Japan) are put in a stainless beaker,followed by stirring for 1 hr under heating at 120° C. by use of an oilbath, and thereby a melt body in which a resin, a pigment and a chargecontrol agent are evenly contained is prepared. The obtained melt isgradually cooled to room temperature with stirring, followed by furtheradding 100 parts by weight of NORPAR 15 (trade name, manufactured byExxon Corp.,).

As a system temperature goes down, mother particles that include thepigment and the charge control agent and have particle diameters in therange of 10 to 20 μm are precipitated. Then, 100 g of precipitatedmother particles are put in a 01 type attritor and pulverized by use ofsteel balls having a diameter of 0.8 mm. The pulverization is continued,while a volume average particle diameter is monitored by use of acentrifugal sedimentation particle size distribution analyzer (tradename: SA-CP4L, manufactured by Shimadzu Corporation), until a particlediameter of 2.5 μm is obtained.

In the next place, 20 parts of the obtained concentrated particles(particle concentration 18% by weight) are diluted, so that a particleconcentration may be 2% by weight with respect to a particle dispersionliquid, with 160 parts by weight of eicosane (C₂₀H₄₂, melting point:36.8° C.) previously heated and melted at 75° C., followed by sufficientstirring.

The obtained particle dispersion liquid is repeatedly subjected 5 timesto suction filtering/water washing, and thereby, magenta colorelectrophoretic particle B is obtained. An average particle diameter ofthe electrophoretic particles B is 1 μm and the charge polarity insilicone oil is positive polarity.

<Preparation of Electrophoretic Particle C> —Preparation of Dispersionliquid A—

-   Cyclohexyl methacrylate: 53 parts by weight-   Magenta pigment (trade name: Carmine 6B, manufactured by    Dainichiseika Color & Chemicals Incorporated): 3 parts by weight-   Charge control agent (trade name: COPY CHARGE PSY VP2038,    manufactured by Clariant Japan): 2 parts by weight

A mixture of the above-described components is pulverized by use of aball mill with zirconia balls having a diameter of 10 mm for 20 hr, andthereby a dispersion liquid A is prepared.

—Preparation of Dispersion liquid B—

-   Calcium carbonate: 40 parts by weight-   Water: 60 parts by weight

A mixture of the above-described components is finely pulverized by useof a ball mill, and, thereby a dispersion liquid B is prepared.

—Preparation of Mixture Liquid C—

-   Aqueous solution of 2% by weight of Cellogen: 4.3 g-   Dispersion liquid B: 8.5 g-   Aqueous solution of 20% by weight of sodium chloride: 50 g

The above-described components are mixed, followed by deaerating with anultrasonic device for 10 min, further followed by stirring by use of anemulsifier, and thereby a mixture liquid C is prepared.

—Preparation of Particle—

At first, 35 g of the dispersion liquid A, 1 g of divinyl benzene and0.35 g of a polymerization initiator AIBN (trade name,2,2′-azobisisobutyronitrile) are sufficiently mixed and deaerated for 10min by use of an ultrasonic device. This is put into a mixture liquid C,followed by emulsifying by use of an emulsifier.

In the next place, the obtained emulsified liquid is put in a bottle andsealed with a silicone cap, followed by, by use of a syringe needle,thoroughly depressurizing and deaerating, further followed by filling anitrogen gas, still further followed by reacting at 60° C. for 10 hr toprepare particles. The obtained fine powder is dispersed inion-exchanged water, followed by decomposing calcium carbonate with anaqueous solution of 1 N hydrochloric acid, further followed byfiltering. Thereafter, the filtered product is washed with sufficientamount of distilled water, a particle diameter is regulated, followed bydrying.

The obtained electrophoretic particle C has a magenta color and a volumeaverage particle diameter of 1 μm. Furthermore, in silicone oil, thecharging property is a positive polarity.

<Preparation of Electrophoretic Particle Holding Particle A>

A mixture constituted of 80 parts by weight of methyl methacrylatemonomer, 17 parts by weight of titanium oxide (trade name: TAIPEKU CR63,manufactured by Ishihara Sangyo Kaisha, Ltd.) and 3 parts by weight ofhollow particles (trade name: SX866 (A), manufactured by JSR Corp.) ispulverized for 20 hr by use of a ball mill with zirconia balls of 10mmφ, and thereby a dispersion liquid B-1 is obtained.

A mixture of 40 parts by weight of calcium carbonate and 60 parts byweight of water is operated similarly to the dispersion liquid B-1 andthereby a dispersion liquid B-2 is obtained.

Then, 8.5 parts by weight of the dispersion liquid B-2 and 50 parts byweight of an aqueous solution of 20% sodium chloride are stirred andmixed, and thereby a mixture liquid B-3 is obtained.

In the next place, 35 parts by weight of the dispersion liquid B-1, 1part by weight of dimethacrylic acid ethylene glycol and 0.35 parts byweight of azoisobutylonitrile are mixed, followed by adding the mixtureliquid B-3 to emulsify, and thereby an emulsified liquid B-4 isobtained.

Then, the obtained emulsified liquid B-4 is heated at 65° C. undernitrogen gas flow, followed by stirring for 15 hr, and thereby solidparticles B-5 are obtained.

To the obtained solid particles B-5, 15 parts by weight of 35%hydrochloric acid are added, followed by stirring, further followed by,after calcium carbonate is dissolved, repeating suction filtering/waterwashing 5 times, and thereby white particles B-6 are obtained. Theobtained white particles B-6 are sieved with a filter, and, thereby,white particles having an average particle diameter of 13 μm and chargednegatively in silicone oil (electrophoretic particle holding particlesA) are obtained.

<Preparation of Reflective Member A>

The white particles B-6 obtained when the electrophoretic particleholding particles A are prepared are further surface treated with asilane coupling agent γ-APS (trade name: KBM903, manufactured byShin-Etsu Chemical Co., Ltd.) to control the charging properties,followed by, similarly to a case where the electrophoretic particleholding particles A are prepared, classifying with a filter, and therebywhite particles having an average particle diameter of 13 μm andpositively charged in silicone oil are obtained.

Example 1

A display medium having a configuration shown in FIG. 8 is preparedaccording to a procedure below.

In the beginning, on one surface of a transparent substrate constitutedof 50 mm×50 mm and 0.7 mm thick glass, as a transparent electrode, anITO film is formed at a thickness of 50 nm by a sputtering method.

On the other hand, on one surface of a back substrate constituted of 50mm×50 mm and 0.7 mm thick alumina ceramics, as a back electrode, copperis formed at a film thickness of 500 nm by a sputtering method.Subsequently, on a surface of the back substrate at a side where theback electrode is disposed, an epoxy resin (trade name: SU-8,manufactured by MicroChem Corp.) is coated, followed by exposing and wetetching, and thereby a partition wall having a height of 100 μm and awidth of 20μm is formed along an outer circumference of the backsubstrate.

Subsequently, on a top portion of the partition wall, a hot-melt typeepoxy adhesive is coated and formed, followed by evenly filling theelectrophoretic particle holding particles A in partitioned portions bya partition wall on the back substrate and by filling a dispersionliquid in which the electrophoretic particles A are dispersed insilicone oil (trade name: KF-96, manufactured by Shin-Etsu ChemicalCorp.) (solid content: 3% by volume) up to a height of the partitionwall.

Finally, a surface of the transparent substrate on which the transparentelectrode is formed and the partition wall disposed on the backsubstrate are brought into close contact with each other so that air isnot included in a gap between the transparent substrate and the backsubstrate, followed by adhering under heating, and thereby a displaymedium is prepared.

With thus prepared display medium, from a side of the display medium atwhich the transparent substrate is disposed, the display medium isobserved to confirm whether or not a halftone can be displayed andwhether or not it has the memory property.

In the beginning, a voltage of 20 V is applied to both electrodes sothat a transparent electrode side of a display medium immediately afterassembling may be minus until a display density of a display surface issufficiently saturated. At this time, a deep red color is displayed onthe display surface.

Subsequently, when, with a transparent electrode side set plus, whilethe voltage is gradually raised from 0 V, a voltage is applied to bothelectrodes for a sufficient time period at the respective voltagevalues, at voltages of 20 V or higher, the display density of thedisplay surface varies from a deep red color to a complete white color(minimum density).

In the next place, after a deep red color (maximum density) is displayedon a display surface, an electric field is applied once at the minimumvoltage value (5 V) where the display density of the display surfacebegins varying, and variations in the display density and display colorat this time are confirmed. At this time, the evaluation is carried outwhile a time during which an electric field is applied is graduallyshortened.

As the result, it is confirmed that, when an application time is 0.5 secor less, the display density of the display surface after an electricfield has been applied, without varying from a deep red color to acomplete white color (minimum density), exhibits a thin red color andthis state is maintained. From the above, it is found that the displaymedium can display a halftone and has (an absolute value of) a thresholdvoltage of 5 V and that, since an inter-electrode distance is 100μm, (anabsolute value of) a threshold value of an electric field is 500 V/cm.

—Evaluation of Memory Property—

In the next place, the sustainability of the display state (memoryproperty) is evaluated at three levels of the maximum density (deep redcolor), the minimum density (complete white color) and an intermediatedensity (thin red color).

When the memory property is evaluated, after a voltage is applied and adisplay state is set at the maximum density, the minimum density or theintermediate density, the voltage application is stopped, and thereflectance of the display surface at that time is obtained by use ofX-Rite 404 (trade name, manufactured by X-Rite Corp.) and taken as aninitial reflectance. Furthermore, the reflectance of the display surfaceafter leaving for one day after the voltage application is stopped ismeasured as a reflectance after leaving and evaluated according tocriteria below. Results are shown in Table 1.

The intermediate density is a display state when, from a statedisplaying the maximum density, with a transparent electrode side setplus, a voltage corresponding to the threshold voltage is applied for apredetermined time period (a half a time necessary for a display stateto vary from the maximum density to the minimum density when an electricfield is applied at a threshold voltage).

A: |100×reflectance after leaving/initial reflectance| is in the rangeof 98% or higher but 100% or lower.

B: |100×reflectance after leaving/initial reflectance| is in the rangeof 95% or higher but lower than 98%.

C: |100×reflectance after leaving/initial reflectance| is in the rangeof 85% or higher but lower than 95%.

D: |100×reflectance after leaving/initial reflectance| is less than 85%.

Example 2

Except that, in example 1, in place of a dispersion liquid whereelectrophoretic particles A are dispersed in silicone oil (trade name:KF-96, manufactured by Shin-Etsu Chemical Co., Ltd.), a dispersionliquid where electrophoretic particles B are dispersed in silicone oil(trade name: KF-96, manufactured by Shin-Etsu Chemical Co., Ltd.) isused, similarly to example 1, a display medium is prepared.

Subsequently, when, similarly to example 1, whether or not a halftonecan be displayed is evaluated, it is confirmed that a halftone can bedisplayed.

Furthermore, it is found that (an absolute value of) a threshold voltagein the display medium of example 2 is 200 V and (an absolute value of) athreshold value of an electric field, since an inter-electrode distanceis 100 μm, is 20 kV/cm. Still further, when an electric field is appliedat a threshold voltage, a time necessary for the display state to varyfrom the maximum density to the minimum density is 0.2 sec. Furthermore,a result of evaluation of the memory property is shown in Table 1.

Example 3

Except that, in example 1, in place of a dispersion liquid whereelectrophoretic particles A are dispersed in silicone oil (trade name:KF-96, manufactured by Shin-Etsu Chemical Co., Ltd.), a dispersionliquid where electrophoretic particles C are dispersed in silicone oil(trade name: KF-96, manufactured by Shin-Etsu Chemical Co., Ltd.) isused, similarly to example 1, a display medium is prepared.

Subsequently, when, similarly to example 1, whether or not a halftonecan be displayed is evaluated, it is confirmed that a halftone can bedisplayed.

Furthermore, it is found that (an absolute value of) a threshold voltagein the display medium of example 3 is 1 V and (an absolute value of) athreshold value of an electric field, since an inter-electrode distanceis 100 μm, is 100 V/cm. Still further, when an electric field is appliedat a threshold voltage, a time necessary for the display state to varyfrom the maximum density to the minimum density is 1 sec. Furthermore, aresult of evaluation of the memory property is shown in Table 1.

Referential Example 1

Except that, in example 1, in place of the electrophoretic particleholding particles A, a reflective member A is used, similarly to example1, a display medium is prepared.

Subsequently, when similarly to example 1 whether or not a halftone canbe displayed is evaluated, it is confirmed that the halftone cannot bedisplayed. Furthermore, when the display states are the maximum densityand the minimum density, similarly to example 1, the memory property isevaluated and results are shown in Table 1.

TABLE 1 Absolute Value of Threshold Constitution Value of MemoryProperty of Display Electric Halftone Maximum Intermediate MinimumMedium Field (V/cm) Display Density Density Density Example 1 FIG. 8 500Available A A A Example 2 FIG. 8 20k Available A A A Example 3 FIG. 8100 Available B B B Referential *1 — Non C-D — C-D Example 1 Available*1: In a display medium shown in FIG. 8, in place of electrophoreticparticle holding particles, a reflective member is used.

Example 4 <Preparation of Electrophoretic Particle D>

As electrophoretic particles D, pigment base particles (content ofcoloring agent: 8% by weight, average particle diameter: 0.3 μm, magentacolor) are prepared as shown below.

A mixture of 40 parts by weight of an ethylene/methacrylic acidcopolymer (trade name: NEWCREL N699, manufactured by DUPONT Corp.,copolymerization ratio (molar ratio) of ethylene/methacrylicacid=89/11), 8 parts by weight of a magenta pigment (trade name: CARMINE6B, manufactured by Dainichiseika Color & Chemicals, Incorporated) and 2parts by weight of a charge control agent (trade name: COPY CHARGE PSYVP2038, manufactured by Clariant Japan) is put in a stainless beaker,followed by stirring for 1 hr under heating at 120° C. by use of an oilbath, and thereby a melt where a resin, a pigment and a charge controlagent are evenly contained is prepared. An obtained melt is graduallycooled to room temperature with stirring, followed by further adding 100parts by weight of NORPAR 15 (trade name, manufactured by Exxon Corp.).

As a system temperature goes down, mother particles that include thepigment and the charge control agent and have a particle diameter in therange of 10 to 20μm are precipitated. Then, 100 g of the precipitatedmother particles is put in a 01 type attritor and pulverized by use ofsteel balls having a diameter of 0.8 mm. The pulverization is continued,while a volume average particle diameter is monitored by use of acentrifugal sedimentation particle size distribution analyzer (tradename: SA-CP4L, manufactured by Shimadzu Corporation), until a particlediameter of 1.2 μm is obtained.

In the next place, 20 parts of the obtained concentrated particles(particle concentration: 18% by weight) are diluted, so that a particleconcentration may be 2% by weight with respect to a particle dispersionliquid, with 160 parts by weight of eicosane (C₂₀H₄₂, melting point:36.8° C.) previously heated and melted at 75° C., followed by thoroughstirring.

The obtained particle dispersion liquid is repeatedly subjected 5 timesto suction filtering/water washing, and thereby, magenta colorelectrophoretic particles D are obtained. An average particle diameterof the electrophoretic particles D is 0.3 μm and the charged polarity insilicone oil is positive polarity.

<Preparation of White Particle>

As white particles, polymer particles (content of titanium oxide(white-coloring agent): 40 to 70%, primary component: polymethylacrylate, average particle diameter: 5 μm) are prepared as follows.

A mixture constituted of 80 parts by weight of methyl methacrylatemonomer, 17 parts by weight of titanium oxide (trade name: TAIPEKU CR63,manufactured by Ishihara Sangyo Kaisha, Ltd.) and 3 parts by weight ofhollow particles (trade name: SX866 (A), manufactured by JSR Corp.) ispulverized for 20 hr by use of a ball mill with zirconia balls of 10mmφ, and thereby a dispersion liquid D-1 is obtained.

A mixture of 40 parts by weight of calcium carbonate and 60 parts byweight of water is operated similarly to the dispersion liquid D-1 andthereby a dispersion liquid D-2 is obtained.

Then, 8.5 parts by weight of the dispersion liquid D-2 and 50 parts byweight of an aqueous solution of 20% sodium chloride are stirred andmixed, and thereby a mixture liquid D-3 is obtained.

In the next place, 35 parts by weight of the dispersion liquid D-1, 1part by weight of dimethacrylic acid ethylene glycol and 0.35 parts byweight of azoisobutylonitrile are mixed, followed by adding the mixtureliquid D-3 to emulsify, and thereby an emulsified liquid D-4 isobtained.

Then, the obtained emulsified liquid D-4 is heated at 65° C. undernitrogen gas flow, followed by stirring for 15 hr, and thereby solidparticles D-5 are obtained.

To the obtained solid particles D-5, 15 parts by weight of 35%hydrochloric acid are added, followed by stirring to dissolve calciumcarbonate, further followed by repeating suction filtering/water washing5 times, and thereby white particles D-6 are obtained. The obtainedwhite particles D-6 are sieved with a filter, and, thereby, whiteparticles having an average particle diameter of 5 μm and negativelycharged in silicone oil (electrophoretic particle holding particles D)are obtained.

<Preparation of Electrophoretic Particle Holder D>

As an electrophoretic particle holder D, TORAYMICRON EM020 (trade name,manufactured by Toray Industries, Inc., weight: 20 g/m², thickness: 150μm) that is an electret-type non-woven fabric is prepared.

A display medium having a configuration shown in FIG. 9 is preparedaccording to a procedure below.

In the beginning, on one surface of a transparent substrate constitutedof 50 mm×50 mm glass having a thickness of 0.7 mm, as a transparentelectrode, an ITO film is formed at a thickness of 50 nm by a sputteringmethod.

On the other hand, on one surface of a back substrate constituted of 50mm×50 mm alumina ceramics having a thickness of 0.7 mm, as a backelectrode, copper is formed at a film thickness of 500 nm by asputtering method. Subsequently, a region of 1 cm² on a surface of theback substrate at a side where the back electrode is disposed ispartitioned with a polyimide tape (trade name: No. 360, produced byNitto Denko Co., Ltd.) and thereby a gap having a height of 120 μm isformed.

Subsequently, on a top portion of the polyimide tape, a hot-melt typeepoxy adhesive is coated and formed, followed by cutting TORAYMICRONEM020 into 1 cm² and disposing as the electrophoretic particle holder Din a region partitioned by a partition wall on the back substrate.

When, on the disposed TORAYMICRON EM020 (hereinafter, referred to asEM020), 60 μl of a dispersion liquid in which the above-preparedelectrophoretic particle D and white particle, respectively, aredispersed at a content of 8% by weight and a content of 5% by weight insilicone oil (KF-96, manufactured by Shin-Etsu Chemical Co., Ltd.) isadded dropwise, the dispersion liquid evenly spreads over the EM020.

Finally, a surface of the transparent substrate at which the transparentelectrode is formed and the partition wall disposed on the backsubstrate are brought into close contact with each other so that air maynot come into a gap between the transparent substrate and the backsubstrate, followed by sealing a circumference with a UV-curable resin(trade name: 3003, manufactured by ThreeBond Co., Ltd.), and thereby adisplay medium is prepared.

With thus prepared display medium, from a side of the display medium atwhich the transparent substrate is disposed, the display medium isobserved to confirm whether or not a halftone can be displayed andwhether or not it has the memory property similarly as in example 1.

In the beginning, a voltage is applied to both electrodes so that atransparent electrode side of a display medium immediately afterassembling may be plus for 10 sec at each of voltage values with thevoltage being raised stepwise by 20 V from 0 V. The voltage is applieduntil the display density of the display surface is sufficientlysaturated at the application of a voltage of the respective voltagevalues. At this time, when a state of white particles adhered to theEM020 immediately after application of a voltage of each of values for10 sec is observed with a microscope (trade name: VHX DEGITALMICROSCOPE, manufactured by Keyence Corporation, magnification: 500times), it is confirmed that, immediately after application of a voltageof 100 V or lower, white particles are stuck same as an initial state toa surface of fibers of EM020 disposed in the display medium and whiteparticles adhered to a region within 20 μm from the transparentelectrode do not increase even when a voltage is applied. Furthermore,it is confirmed that, immediately after a voltage of 200 V or higher isapplied, almost all of the white particles in the display medium arestuck to a region of EM020 within 20 μm from the transparent electrode.This is considered that a force of moving white particles due toapplication of a voltage is larger than a magnitude of an interactionbetween a white particle and the fiber constituting EM020.

In the next place, with a transparent electrode side set minus, avoltage of −300 V is applied to both electrodes for 20 sec to display adeep red color (maximum density) on a display surface, and then, anelectric field is once applied at the minimum voltage value (250 V)where the display density of the display surface starts varying, and thevariations in the display density and display color at this time areconfirmed. At this time, while a time during which an electric field isapplied is gradually shortened, the evaluation is carried out.

As a result, it is confirmed that, when an application time is 3 sec orless, a display density of a display surface after an electric field hasbeen applied, without changing from a deep red color to a complete whitecolor (minimum density), exhibits a thin red color and the state ismaintained. From this, it is found that the display medium can display ahalftone, (an absolute value of) a threshold voltage of theelectrophoretic particles D that are magenta particles is 250 V and (anabsolute value of) a threshold value of an electric field, since aninter-electrode distance is 120 μm, is 21 kV/cm.

—Evaluation of Memory Property—

In the next place, when, similarly to example 1, whether or not thehalftone can be displayed is evaluated, it is confirmed that thehalftone can be displayed.

In example 4, a threshold voltage of the electrophoretic particles Dfilled in the display medium is 250 V and a threshold voltage of thewhite particles is 180 V. Accordingly, it is found that (absolute valuesof) threshold values of electric fields, since an inter-electrodedistance is 120 μm, are 21 kV/cm and 15 kV/cm, respectively.Furthermore, when electric fields are applied at the respectivethreshold voltages, times necessary for a display state to change fromthe maximum density to the minimum density are 5 sec and 8 sec,respectively.

Threshold voltages of the respective electrophoretic particles D andwhite particles are measured by preparing and measuring display media ofthe respective configurations of a configuration containing theelectrophoretic particles D and the white particles and a configurationcontaining the white particles but not containing the electrophoreticparticles D in the display media prepared in example 4. Results ofevaluation of the memory property are shown in Table 2.

Example 5 <Preparation of Electrophoretic Particle E>

As electrophoretic particles E, polymer particles (content of coloringagent: 20% by weight, average particle diameter: 5 μm, magenta color)are prepared as shown below.

A mixture constituted of 80 parts by weight of methyl methacrylatemonomer, 5 parts by weight of a magenta pigment (trade name: CARMINE 6B,manufactured by Dainichiseika Color & Chemicals, Incorporated) and 3parts by weight of hollow particle (trade name: SX866 (A), manufacturedby JSR Corp.) is subjected to ball mill pulverization with zirconiaballs of 10 mmφ for 20 hr, and thereby a dispersion liquid E-1 isobtained.

A mixture of 40 parts by weight of calcium carbonate and 60 parts byweight of water is operated similarly to the dispersion liquid E-1 andthereby a dispersion liquid E-2 is obtained.

Then, 8.5 parts by weight of the dispersion liquid E-2 and 50 parts byweight of an aqueous solution of 20% sodium chloride are stirred andmixed, and thereby a mixture liquid E-3 is obtained.

In the next place, 35 parts by weight of the dispersion liquid E-1, 1part by weight of dimethacrylic acid ethylene glycol and 0.35 parts byweight of azoisobutyl nitrile are mixed, followed by adding the mixtureliquid E-3 to emulsify, and thereby an emulsified liquid E-4 isobtained.

Then, the obtained emulsified liquid E-4 is heated at 65° C. undernitrogen gas flow, followed by stirring for 15 hr, and thereby solidparticles E-5 are obtained.

To the obtained solid particles E-5, 15 parts by weight of 35%hydrochloric acid are added, followed by stirring to dissolve calciumcarbonate, further followed by repeating suction filtering/water washing5 times, and thereby particles E-6 are obtained. The obtained particlesE-6 are sieved with a filter, and, thereby, magenta color particleshaving an average particle diameter of 5μm and charged negative insilicone oil (electrophoretic particle holding particles E) areobtained.

<Preparation of Electrophoretic Particle F>

As electrophoretic particles F, polymer particles (content of coloringagent: 20% by weight, average particle diameter: 13 μm, cyan color) areprepared as shown below.

A mixture constituted of 80 parts by weight of methyl methacrylatemonomer, 5 parts by weight of a cyan pigment (trade name: COPPERPHTHALLOCYANINE BLUE, manufactured by Dainichiseika Color & Chemicals,Incorporated) and 3 parts by weight of hollow particles (trade name:SX866 (A), manufactured by JSR Corp.) is subjected to ball millpulverization with zirconia balls of 10 mmφ for 20 hr, and thereby adispersion liquid F-1 is obtained.

A mixture of 40 parts by weight of calcium carbonate and 60 parts byweight of water is operated similarly to the dispersion liquid F-1 andthereby a dispersion liquid F-2 is obtained.

Then, 8.5 parts by weight of the dispersion liquid F-2 and 50 parts byweight of an aqueous solution of 20% sodium chloride are stirred andmixed, and thereby a mixture liquid F-3 is obtained.

In the next place, 35 parts by weight of the dispersion liquid F-1, 1part by weight of dimethacrylic acid ethylene glycol and 0.35 parts byweight of azoisobutyl nitrile are mixed, followed by adding the mixtureliquid F-3 to emulsify, and thereby an emulsified liquid F-4 isobtained.

Then, the obtained emulsified liquid F-4 is heated at 65° C. undernitrogen gas flow, followed by stirring for 15 hr, and thereby solidparticles F-5 are obtained.

To the obtained solid particles F-5, 15 parts by weight of 35%hydrochloric acid are added, followed by stirring to dissolve calciumcarbonate, further followed by repeating suction filtering/water washing5 times, and thereby particles F-6 are obtained. The obtained particlesF-6 are sieved with a filter, and, thereby, cyan colored particleshaving an average particle diameter of 13 μm and charged negatively insilicone oil (electrophoretic particle holding particles F) areobtained.

Using the above-prepared electrophoretic particles E and F as theelectrophoretic particles, and using TORAYMICRON EM020 used in theexample 4 as an electrophoretic particle holder, a display medium isprepared similarly to example 4.

The electrophoretic particles E and the electrophoretic particles F,respectively, are dispersed in silicone oil (trade name: KF-96,manufactured by Shin-Etsu Chemical Co., Ltd.) at a content of 5% byweight and a content of 5% by weight to prepare a dispersion liquid and60 μl of the dispersion liquid is added dropwise to EM020 similarly toexample 4.

With the thus prepared display medium, similarly to example 4, from aside of the display medium at which the transparent substrate isdisposed, the display medium is observed to confirm whether or not ahalftone can be displayed and whether or not it has the memory property.

In the beginning, a voltage is applied to both electrodes so that atransparent electrode side of a display medium immediately afterassembling may be plus for 20 sec at each of voltage values with thevoltage being raised stepwise by 20 V from 0 V. When, in this state, avoltage is applied at each of the voltage values until a display densityof a display surface is saturated, the electrophoretic particles E andelectrophoretic particles F, respectively, are observed to move toward adisplay surface side (transparent electrode side). At this time, whenstates of the electrophoretic particles E and the electrophoreticparticles F adhered to the EM020 immediately after application of avoltage at each of values for 20 sec are observed with a microscope(trade name: VHX DIGITAL MICROSCOPE, manufactured by KeyenceCorporation, magnification: 500 times), immediately after application ofa voltage of 50 to 150 V, the electrophoretic particles F (averageparticle diameter: 13 μm, cyan color) are confirmed to be stuck in aregion within 20 μm from a transparent electrode of EM020 disposed inthe display medium and a display surface of the display medium has acyan color.

Furthermore, it is confirmed that, immediately after a voltage of 200 Vor higher is applied, almost all of the electrophoretic particles F andelectrophoretic particles E (average particle diameter: 5 μm, magentacolor) are stuck in a region within 20 μm from the transparent electrodeof the EM020 disposed in the display medium, and a display surface ofthe display medium has a violet color.

In the next place, when a voltage of 100 V is applied to both electrodesfor 20 sec with a transparent electrode side set minus, theelectrophoretic particles F move toward a back substrate side to displaya deep red color (magenta color)(maximum density). Even after a voltageapplication is stopped, a color of the display surface does not show aremarkable change.

From this, it can be said that owing to a display medium of example 5that contains the electrophoretic particles E and the electrophoreticparticles F, which are different from each other in an absolute value ofa threshold value of an electric field, a display medium capable of acolor display can be provided. The difference in an absolute value of athreshold value of an electric field is considered to be derived fromthe difference between an electrostatic interaction between theelectrophoretic particles F and fibers that constitute the EM020 and anelectrostatic interaction between the electrophoretic particles E andfibers that constitute the EM020. It is considered that, although theelectrophoretic particles F having an average particle diameter of 13 μmare larger in an absolute value of a charge amount than that of theelectrophoretic particles E having an average particle diameter of 5 μm,an average distance from a fiber that constitutes the EM020 is larger;accordingly, the electrophoretic particles F is smaller in a force heldby the EM020 than the electrophoretic particles E, but a moving forceowing to an electric field formed between the substrates increases inproportion to a charge amount, and thus an absolute value of thethreshold value of the electric field of the electrophoretic particles Fis smaller than that of the electrophoretic particle E.

In the next place, after a deep red color (magenta color) is displayedas the maximum density on the display surface, a voltage is appliedbetween both electrodes with the transparent electrode side set minusand gradually raised. An electric field is applied once at the smallestvoltage value (180 V) where the display density of a display surfacebegins varying, and variations in a display density and a display colorat this time are confirmed. At this time, while an electric fieldapplying time is gradually shortened, evaluation is carried out.

As a result, it is confirmed that, when an application time is 5 sec orless, the display density of the display surface after an electric fieldhas been applied, without varying from a deep red color to a completewhite color (minimum density), exhibits a thin red color and this stateis maintained. From the above, it is found that the display medium candisplay a halftone and has (an absolute value of) a threshold voltage ofthe electrophoretic particles E that are magenta colored particles of180 V and, since an inter-electrode distance is 120 μm, (an absolutevalue of) a threshold value of an electric field of 15 kV/cm.Furthermore, since (an absolute value of) a threshold voltage of thecyan electrophoretic particles F is 80 V and an inter-electrode distanceis 120 μm, (an absolute value of) a threshold value of an electric fieldis found to be 7 kV/cm.

—Evaluation of Memory Property—

Subsequently, when, similarly to example 1, whether or not a halftonecan be displayed is evaluated, it is confirmed that a halftone can bedisplayed.

In example 5, among the electrophoretic particles E (magenta color) andthe electrophoretic particles F (cyan color) that are filled in adisplay medium, with a state where only the magenta electrophoreticparticles E are present at a display surface side being taken as themaximum density and a state where all of the electrophoretic particles Eand F move toward the back substrate side being taken as the minimumdensity, the memory property is evaluated. A result of evaluation of thememory property is shown in Table 2.

Example 6

Using the electrophoretic particles E and F prepared in example 5 as theelectrophoretic particles, and using ELTASAQUA PA3023 (trade name,produced by ASAHI KASEI FIBERS Corporation, weight: 23 g/m², thickness:200 μm) in place of TORAYMICRON EM020 used in the example 5, similarlyto example 5, a display medium is prepared.

A height of a partition wall of the display medium is set to 200 μm inaccordance with a thickness of the ELTASAQUA PA3023.

Using the thus prepared display medium, similarly to example 6, thedisplay medium is observed from a side of the display medium at which atransparent substrate is disposed to confirm whether or not a halftonecan be displayed and whether or not it has the memory property.

In the beginning, a voltage is applied to both electrodes so that atransparent electrode side of a display medium immediately afterassembling may be plus for 10 sec at each of voltage values with thevoltage being raised stepwise by 10 V from 0 V. When, in this state, avoltage is applied at each of the voltage values until a display densityof a display surface is sufficiently saturated, the electrophoreticparticles E and electrophoretic particles F, respectively, are observedto move toward a display surface side (transparent electrode side). Atthis time, when states of the electrophoretic particles E and theelectrophoretic particles F adhered to the EM020 immediately afterapplication of a voltage at each of values for 10 sec are observed witha microscope (trade name: VHX DIGITAL MICROSCOPE, manufactured byKeyence Corporation, magnification: 500 times), immediately afterapplication of a voltage of 10 to 50 V, the electrophoretic particles E(average particle diameter: 5 μm, magenta color) are confirmed to bestuck in a region within 20 μm from a transparent electrode of EM020disposed in the display medium and a display surface of the displaymedium has a magenta color.

Furthermore, it is confirmed that, immediately after a voltage of 100 Vor higher is applied, almost all of the electrophoretic particles E andelectrophoretic particles F (average particle diameter: 13 μm, cyancolor) are stuck in a region within 20 μm from the transparent electrodeof the EM020 disposed in the display medium, and a display surface ofthe display medium has a violet color.

Thus, when, contrary to example 4, an applied voltage is graduallyraised, it is observed that, among the electrophoretic particles E andF, the electrophoretic particles E firstly move toward the displaysurface side. When the voltage application time at each of voltages inthe range of 10 to 50 V is lengthened (30 sec) more than the above, bothof the electrophoretic particles E and F move toward the display surfaceside.

This shows that the electrophoresis of the electrophoretic particles isaffected by a distance between fibers that constitute an electrophoreticparticle holder and a particle diameter of the electrophoretic particlesheld by the electrophoretic particle holder. That is, it is consideredthat, relative to a distance between fibers that constitute ELTASAQUAPA3023 as the electrophoretic particle holder, an average particlediameter of the electrophoretic particles E (average particle diameter:5 μm) is smaller than an average particle diameter of theelectrophoretic particle F (average particle diameter: 13 μm);accordingly, the electrophoretic particles E can sufficiently movewithin the electrophoretic particle holder but the electrophoreticparticles F are suppressed in the electrophoresis in the electrophoreticparticle holder.

A charge amount-of the ELTASAQUA PA3023 that is used as theelectrophoretic particle holder in example 6 is so small as not toaffect on the electrophoresis of the electrophoretic particle.Accordingly, in the case of example 6, it is considered that a chargeamount of the electrophoretic particle holder (ELTASAQUA PA3023) affectson the electrophoresis of the electrophoretic particles only slightly.

The adherence states of the electrophoretic particles E and F to theelectrophoretic particle holder in example 6 (ELTASAQUA PA3023) aresmaller than that to the electrophoretic particle holder in example 5(TORAYMICRON EM020); accordingly, a situation of being relatively freelyelectrophoresed between fibers can be observed. Furthermore, thesustainability of a color showed on a display surface after a voltage isapplied in example 6 is inferior to that of example 5.

In the next place, a voltage of 50 V is applied to both electrodes for10 sec so that a transparent electrode side may be plus and thereby onlymagenta color electrophoretic particles E of the electrophoreticparticles F and E are moved toward the display substrate side to displaya deep red color (magenta color) (maximum density) on a display surface.Even after a voltage application is stopped, a color on the displaysurface does not remarkably vary.

In the next place, a voltage is applied to both electrodes so that atransparent electrode side may be minus and gradually raised, anelectric field is only once applied at the smallest voltage value (35 V)where a display density of a display surface starts varying, andvariations in the display density and display color at this time areconfirmed. At this time, with a time during which an electric field isapplied being gradually shortened, the evaluation is carried out.

As the result thereof, it is confirmed that, when an application time is7 sec or less, a display density of the display surface after anelectric field has been applied, without varying from a deep red to acomplete white (minimum density), shows a thin red color and this statecan be sustained. From this, it is found that the display medium candisplay a halftone, (an absolute value of) a threshold voltage of theelectrophoretic particles E that are magenta particles is 35 V and (anabsolute value of) a threshold value of an electric field, since aninter-electrode distance is 200 μm, is 1700 V/cm. Furthermore, it isfound that (an absolute value of) a threshold voltage of theelectrophoretic particles F that are cyan particles is 50 V and (anabsolute value of) a threshold value of an electric field, since aninter-electrode distance is 200 μm, is 2500 V/cm.

—Evaluation of Memory Property—

Subsequently, when, similarly to example 1, whether or not a halftonecan be displayed is evaluated, it is confirmed that a halftone can bedisplayed.

In example 6, among the electrophoretic particles E (magenta color) andthe electrophoretic particles F (cyan color), which are filled in adisplay medium, with a state where only the magenta electrophoreticparticles E are present at a display surface side being taken as themaximum density and a state where all of the electrophoretic particles Eand F move toward the back substrate side being taken as the minimumdensity, the memory property is evaluated. A result of evaluation of thememory property is shown in Table 2.

Example 7 <Preparation of Electrophoretic Particle Holder>

The ELTASAQUA PA3023, after leaving for 3 sec on a hot plate (tradename: HI-1000, manufactured by As-one Corp.) of which plate temperatureis set at 180° C., is peeled off from the hot plate and left to naturalcooling.

When a surface of the thus heat treated ELTASAQUA PA3023 (hereinafter,referred to as PA3023) is confirmed with a microscope (trade name: VHXDIGITAL MICROSCOPE, manufactured by Keyence Corporation, magnification:250 times), it is confirmed that fibers in the proximity of a surface ofthe PA3023 are slightly dissolved and entangled each other to be denserin the inter-fiber distance in comparison with a region that is not heattreated.

Using the heat-treated PA3023 as the electrophoretic particle holder andusing the electrophoretic particles E and F prepared in example 5 as theelectrophoretic particle, by a method similar to example 6, a displaymedium is prepared.

When a display medium is prepared, the PA 3023 is disposed so that asurface of the heat treated PA3023 at a heat-treated side may come intocontact with an electrode (transparent electrode) at a display surfaceside and thereby a display medium shown in FIG. 21 is prepared.

Subsequently, when, similarly to example 6, whether or not a halftonecan be displayed is evaluated, it is conformed that the halftone can bedisplayed.

In the display medium of example 7, it is found that (an absolute valueof) a threshold voltage of the electrophoretic particles E that aremagenta particles is 45 V and (an absolute value of) a threshold valueof an electric field, since an inter-electrode distance is 200 μm, is2200 V/cm. Furthermore, it is found that (an absolute value of) athreshold voltage of the electrophoretic particles F that are cyanparticles is 60 V and a (absolute value) of a threshold value of anelectric field is, since an inter-electrode distance is 200 μm, 3000V/cm. Still further, when an electric field is applied at the thresholdvoltage, a time necessary for the display state to vary from the maximumdensity to minimum density is 12 sec. An evaluation result of the memoryproperty is shown in Table 2.

TABLE 2 Absolute Value of Threshold Constitution Value of MemoryProperty of Display Electric Field Halftone Maximum Intermediate MinimumMedium (V/cm) Display Density Density Density Example 4 FIG. 9 21kAvailable A A A Example 5 FIG. 9 15k Available B B B Example 6 FIG. 91700 Available C C-D C Example 7 FIG. 21 2200 Available B-C C C

The foregoing description of the exemplary embodiments of the presentinvention has been provided for the purposes of illustration anddescription. It is not intended to be exhaustive or to limit theinvention to the precise forms disclosed. Obviously, many modificationsand variations will be apparent to practitioners skilled in the art. Theexemplary embodiments were chosen and described in order to best explainthe principles of the invention and its practical applications, therebyenabling others skilled in the art to understand the invention forvarious exemplary embodiments and with the various modifications as aresuited to the particular use contemplated. It is intended that the scopeof the invention be defined by the following claims and theirequivalents.

The foregoing description of the embodiments of the present inventionhas been provided for the purposes of illustration and description. Itis not intended to be exhaustive or to limit the invention to theprecise forms disclosed. Obviously, many modifications and variationswill be apparent to practitioners skilled in the art. The embodimentswere chosen and described in order to best explain the principles of theinvention and its practical applications, thereby enabling othersskilled in the art to understand the invention for various embodimentsand with the various modifications as are suited to the particular usecontemplated. It is intended that the scope of the invention be definedby the following claims and their equivalents.

1. A display medium, comprising at least: a pair of substrates, at leastone of the substrates having optical transparency; a dispersion mediumpositioned in a gap between the pair of substrates; one or more kinds ofelectrophoretic particles or two or more kinds of electrophoreticparticles different in color from each other, included in the dispersionmedium; and a holder disposed between the pair of substrates, the holderhaving a function of holding the electrophoretic particles and afunction of controlling, by an external voltage, a movement amount ofthe electrophoretic particles on the holder.
 2. The display medium ofclaim 1, wherein an absolute value of a threshold value of an electricfield at which the electrophoretic particles in a state of being held bythe holder are detached from the holder and moved in the dispersionmedium is in the range of about 100 V/cm to 30 kV/cm.
 3. The displaymedium of claim 1, wherein the electrophoretic particles are chargedwith a positive or negative polarity, and the holder is at leastpartially charged with a polarity opposite to that of all kinds of theelectrophoretic particles.
 4. The display medium of claim 1, wherein theholder includes two or more particulate members.
 5. The display mediumof claim 1, wherein the holder has a color different from that of allkinds of the electrophoretic particles.
 6. The display medium of claim1, wherein the holder includes an aggregate of fibers.
 7. The displaymedium of claim 1, wherein the holder includes a non-woven fabric. 8.The display medium of claim 1, wherein the function of holding theelectrophoretic particles of the holder varies continuously or stepwisein a direction in which the pair of substrates face each other.
 9. Thedisplay medium of claim 1, wherein the function of holding theelectrophoretic particles of the holder decreases continuously orstepwise as a distance from at least one of the pair of substratesincreases in a direction in which the pair of substrates face eachother.
 10. The display medium of claim 1, wherein the holder includes aporous body having gaps through which the electrophoretic particlespass.
 11. The display medium of claim 1, wherein the holder has a whitecolor.
 12. The display medium of claim 1, wherein the electrophoreticparticles include two or more kinds of electrophoretic particlesdifferent from each other in color and in an absolute value of athreshold value of an electric field at which the electrophoreticparticles in a state of being held by the holder are detached from theholder and moved in the dispersion medium.
 13. The display medium ofclaim 1, wherein the electrophoretic particles include two or more kindsof electrophoretic particles different from each other in a volumeaverage primary particle diameter.
 14. The display medium of claim 1,wherein the electrophoretic particles include two or more kinds ofelectrophoretic particles different from each other in a color formed ina state of being dispersed in the dispersion medium and in an absolutevalue of a threshold value of an electric field at which theelectrophoretic particles in a state of being held by the holder aredetached from the holder and moved in the dispersion medium.
 15. Adisplay device, comprising at least: a pair of substrates, at least oneof the substrates having optical transparency; a dispersion mediumpositioned in a gap between the pair of substrates; one or more kinds ofelectrophoretic particles or two or more kinds of electrophoreticparticles different in color from each other, included in the dispersionmedium; a light-modulating layer disposed between the pair of substratesand including a holder having a function of holding the electrophoreticparticles; a pair of electrodes respectively disposed on one substrateside and the other substrate side of the pair of substrates; and anelectric field applicator that is connected to the pair of electrodesand applies an electric field to the dispersion medium, the electricfield applicator controlling a movement amount of the electrophoreticparticles on the holder.
 16. The display device of claim 15, wherein anabsolute value of a threshold value of an electric field at which theelectrophoretic particles in a state of being held by the holder aredetached from the holder and moved in the dispersion medium is in therange of about 100 V/cm to 30 kV/cm; the electric field applicatorapplies, to the dispersion medium, an electric field having a voltagewaveform having cycles each including a first interval where an electricfield is continuously applied at a voltage where an absolute value ofthe voltage is equal to or higher than an absolute value of a thresholdvoltage corresponding to the threshold value of the electric field and asecond interval where an electric field is continuously applied at avoltage where an absolute value of the voltage is less than an absolutevalue of a threshold voltage corresponding to the threshold value of theelectric field; and in the first interval in at least any one of thecycles, the following formula (1) is satisfied:Ep<Emax   Formula (1) wherein Ep denotes a value represented by thefollowing formula (2), and Emax denotes a product of voltage·time (V·s)necessary for varying a display density from the maximum density to theminimum density or from the minimum density to the maximum density whenan electric field is continuously applied to the light-modulating layerat a voltage where an absolute value of the voltage is equal to orhigher than the absolute value of the threshold voltageE _(p)=∫₀ ^(t) ^(p) V(t)dt   Formula (2) wherein t denotes any time (s)within the first interval in the at least any one of the cycles, tpdenotes a time (s) from a start to an end of the first interval in theat least any one of the cycles, and V(t) denotes a voltage (V) at thetime t.
 17. The display device of claim 15, wherein the electrophoreticparticles are charged with a positive or negative polarity, and theholder is at least partially charged with a polarity opposite to that ofall kinds of the electrophoretic particles.
 18. The display device ofclaim 15, wherein the holder includes two or more particulate members.19. The display device of claim 15, wherein the holder has a colordifferent from that of all kinds of the electrophoretic particles. 20.The display device of claim 15, wherein the holder includes an aggregateof fibers.
 21. The display device of claim 15, wherein the holderincludes a non-woven fabric.
 22. The display device of claim 15, whereinthe function of holding the electrophoretic particles of the holdervaries continuously or stepwise in a direction in which the pair ofsubstrates face each other.
 23. The display device of claim 15, whereinthe function of holding the electrophoretic particles of the holderdecreases continuously or stepwise as a distance from at least one ofthe pair of substrates increases in a direction in which the pair ofsubstrates face each other.
 24. The display device of claim 15, whereinthe holder includes a porous body having gaps through which theelectrophoretic particles pass.
 25. The display device of claim 15,wherein the holder has a white color.
 26. The display device of claim15, wherein the electrophoretic particles include two or more kinds ofelectrophoretic particles different from each other in color and in anabsolute value of a threshold value of an electric field at which theelectrophoretic particles in a state of being held by the holder aredetached from the holder and moved in the dispersion medium.
 27. Thedisplay device of claim 15, wherein the electrophoretic particlesinclude two or more kinds of electrophoretic particles different fromeach other in a volume average primary particle diameter.
 28. Thedisplay device of claim 15, wherein the electrophoretic particlesinclude two or more kinds of electrophoretic particles different fromeach other in a color formed in a state of being dispersed in thedispersion medium and in an absolute value of a threshold value of anelectric field at which the electrophoretic particles in a state ofbeing held by the holder are detached from the holder and moved in thedispersion medium.
 29. A display method of switching a display bycarrying out the following processes in any order, the methodcomprising: applying an electric field to a light-modulating layer thatincludes a dispersion medium, one or more kinds of electrophoreticparticles or two or more kinds of electrophoretic particles different incolor from each other, included in the dispersion medium, and a holderhaving a function of holding the electrophoretic particles, the electricfield forming a potential gradient and moving the electrophoreticparticles via the holder, to localize the electrophoretic particles atone side of the light-modulating layer, thereby displaying a color dueto the electrophoretic particles at the one side of the light-modulatinglayer at a maximum density; applying an electric field to thelight-modulating layer, the electric field forming a potential gradientand moving the electrophoretic particles via the holder, to localize theelectrophoretic particles at the other side of the light-modulatinglayer, thereby displaying a color due to the electrophoretic particlesat the one side of the light-modulating layer at a minimum density; andapplying an electric field to the light-modulating layer, the electricfield forming a potential gradient and moving the electrophoreticparticles via the holder, to localize the electrophoretic particlesbetween the one side and the other side of the light-modulating layer,thereby displaying a color due to the electrophoretic particles at theone side of the light-modulating layer at a density smaller than themaximum density but larger than the minimum density.
 30. The displaymethod of claim 29, wherein an absolute value of a threshold value of anelectric field at which the electrophoretic particles in a state ofbeing held by the holder are detached from the holder and moved in thedispersion medium is in the range of about 100 V/cm to 30 kV/cm.
 31. Thedisplay method of claim 29, wherein a voltage waveform of the electricfield applied to the light-modulating layer has cycles each including afirst interval where an electric field is continuously applied at avoltage where an absolute value of the voltage is equal to or higherthan an absolute value of a threshold voltage corresponding to athreshold value of an electric field at which the electrophoreticparticles in a state of being held by the holder are detached from theholder and moved in the dispersion medium, and a second interval wherean electric field is continuously applied at a voltage where an absolutevalue of the voltage is less than an absolute value of a thresholdvoltage corresponding to the threshold value of the electric field, inthe first interval in at least any one of the cycles, the followingformula (3) being satisfied:Ep<Emax   Formula (3) wherein Ep denotes a value represented by thefollowing formula (4), and Emax denotes a product of voltage·time (V·s)necessary for varying a display density from the maximum density to theminimum density or from the minimum density to the maximum density whenan electric field is continuously applied to the light-modulating layerat a voltage where an absolute value of the voltage is equal to orhigher than the absolute value of the threshold voltageE _(p)=∫₀ ^(t) ^(p) V(t)dt   Formula (4) wherein t denotes any time (s)within the first interval in the at least any one of the cycles, tpdenotes a time (s) from a start to an end of the first interval in theat least any one of the cycles, and V(t) denotes a voltage (V) at thetime t.